refactor: return object instead of string from stickers backend

Related to https://github.com/status-im/status-desktop/issues/3725.

Introduce `type PendingTransaction` in `status/types/transaction.nim`. Refactor
`proc buyPack` in `status/stickers.nim` to return an instance of that type
instead of `string`. Eliminate unnecessary threading of `var success` argument
through successive calls in favor of tracking `success` in a field on
`PendingTransaction`.

---

NOTES

Several files in this project have "sticker" in their names or make reference
to "sticker". All those were reviewed as candidates for changes
logically-related to the purpose of this PR:

`status/accounts.nim`
`status/chat.nim`
`status/chat/stickers.nim`
`status/eth/contracts.nim`
`status/eth/stickers.nim`
`status/status.nim`
`status/statusgo_backend/chat.nim`
`status/statusgo_backend/edn_helpers.nim`
`status/stickers.nim`
`status/types/message.nim`
`status/types/pending_transaction_type.nim`
`status/types/setting.nim`
`status/types/sticker.nim`
`status/utils.nim`
`status/wallet/collectibles.nim`

`sendStickerMessage` in `status/statusgo_backend/chat.nim` returns `string` but
refactoring it seems out of scope for this set of changes.

A comment has been left in `status/stickers.nim` re: an additional refactor
that can be made to simplify `trackPendingTransaction` as called by
`buyPack`. The refactor will involve changes in other modules as well.

`type Message` in `status/types/message.nim` has a `sticker*: string` field
that doesn't seem to be made use of by the Nim side of status-desktop, but it
it is made use of by desktop's QML side (see:
`status-desktop/ui/app/AppLayouts/Chat/ChatColumn/ChatMessages.qml`). In the
future it may be preferable to use a different type, or a type alias for
`string`.

`decodeContentHash` in `status/utils.nim` could return an instance of `type
Cid` without converting it to a string, leaving it up to call sites in
status-lib and status-desktop to convert to `string`.

`getStickers` in `status/wallet/collectibles.nim` returns `string`, used by
`{.slot.}` procs in status-desktop (see Nim sources in
`status-desktop/src/app`) that expose the JSON data (as `string`) to
QML. Refactoring the return value seems out of scope for this PR, given ongoing
refactors in status-desktop's front-end architecture.

status/chat/stickers.nim

@@ -1,9 +1,4 @@
 import chronicles
-import ../stickers as status_stickers
+import ../stickers
 
-logScope:
-  topics = "sticker-decoding"
-
-# TODO: this is for testing purposes, the correct function should decode the hash
-proc decodeContentHash*(value: string): string =
-  status_stickers.decodeContentHash(value)
+export decodeContentHash

status/stickers.nim

@@ -76,7 +76,10 @@ proc estimateGas*(packId: int, address: string, price: string, success: var bool
   if success:
     result = fromHex[int](response)
 
-proc buyPack*(self: StickersModel, packId: int, address, price, gas, gasPrice: string, isEIP1559Enabled: bool, maxPriorityFeePerGas: string, maxFeePerGas: string, password: string, success: var bool): string =
+proc buyPack*(self: StickersModel, packId: int, address, price, gas,
+  gasPrice: string, isEIP1559Enabled: bool, maxPriorityFeePerGas: string,
+  maxFeePerGas: string, password: string): PendingTransaction =
+
   var
     sntContract: Erc20Contract
     approveAndCall: ApproveAndCall[100]
@@ -93,9 +96,22 @@ proc buyPack*(self: StickersModel, packId: int, address, price, gas, gasPrice: s
       maxFeePerGas
     )
 
-  result = sntContract.methods["approveAndCall"].send(tx, approveAndCall, password, success)
+  var success: bool
+  let hash = sntContract.methods["approveAndCall"].send(tx, approveAndCall,
+    password, success)
+
+  result = PendingTransaction(hash: hash, success: success,
+    txType: PendingTransactionType.BuyStickerPack)
+
   if success:
-    trackPendingTransaction(result, address, $sntContract.address, PendingTransactionType.BuyStickerPack, $packId)
+    # `proc trackPendingTransaction*` in `statusgo_backend/wallet.nim` can be
+    # refactored to accept an instance of `type PendingTransaction` as its
+    # first argument, allowing its signature to be simplified. However, that
+    # will also require changes re: usage of `trackPendingTransaction` in
+    # `./wallet.nim` and `./ens.nim`, and complementary changes in
+    # status-desktop
+    trackPendingTransaction(result.hash, address, $sntContract.address,
+      result.txType, $packId)
 
 proc getStickerMarketAddress*(self: StickersModel): Address =
   let network = status_settings.getCurrentNetwork().toNetwork()
@@ -147,7 +163,7 @@ proc uninstallStickerPack*(self: StickersModel, packId: int) =
   eth_stickers.saveInstalledStickerPacks(self.installedStickerPacks)
 
 proc decodeContentHash*(value: string): string =
-  result = status_utils.decodeContentHash(value)
+  status_utils.decodeContentHash(value)
 
 proc getPackIdFromTokenId*(tokenId: Stuint[256]): int =
   let network = status_settings.getCurrentNetwork().toNetwork()

status/types/transaction.nim

@@ -5,6 +5,11 @@ import web3/ethtypes, options, stint
 include pending_transaction_type
 
 type
+  PendingTransaction* = ref object
+    hash*: string
+    success*: bool
+    txType*: PendingTransactionType
+
   Transaction* = ref object
     id*: string
     typeValue*: string
@@ -21,8 +26,8 @@ type
     value*: string
     fromAddress*: string
     to*: string
-  
-type 
+
+type
   TransactionData* = object
     source*: Address             # the address the transaction is send from.
     to*: Option[Address]         # (optional when creating new contract) the address the transaction is directed to.
@@ -40,4 +45,4 @@ proc cmpTransactions*(x, y: Transaction): int =
   # Compares first by block number, then by nonce
   result = cmp(x.blockNumber.parseHexInt, y.blockNumber.parseHexInt)
   if result == 0:
-    result = cmp(x.nonce, y.nonce)
+    result = cmp(x.nonce, y.nonce)