Following the work done in
https://github.com/logos-blockchain/lssa/pull/292 and the comment on
extending the work
https://github.com/logos-blockchain/lssa/pull/292#pullrequestreview-3672282664,
this commit introduces a new `--label` option to the `wallet account
new` sub command.
**Usage**:
```
wallet account new public --label "Public test account"
wallet account new private --label "Private test account"
```
Labels have to be unique across all accounts in the wallet storage.
The commit also adds tests, which make use of the `WalletSubCommand`
trait functions (hence the change to make it a `pub trait`).
