Usually, message IDs are generated by their payload and signature and
in receiving nodes calculated in based on the same data as well.
There's no ID attached to messages in-flight.
This turns out to be a bit of a problem for messages that are being
imported from third party systems like discord, as the conversion
and saving of such messages and handling of their possible assets and
attachments are done in separate steps, which changes the message
payloads after their IDs have been generated.
Hence, we're introducing a `ThirdPartyID` property to `common.Message`
and `protobuf.WakuMessage` so receiving nodes of such messages (via the
archive protocol primarily) can easily detect third party/imported
messages and give them special treatment.
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