A0-544: Alerter backup saving

[woocash2]

Backup saving is added into alerter::Service as an intermediary step between handling the data by Handler, and sending the data out to the network so that the cycle of data of type Alert or Multisigned which passes through alerter is like this:

• data gets handled by the Handler which returns some response which is stored in the Service
• data is passed to backup via an appropriate channel
• after some time backup responds with data which triggers a branch in the main select! loop
• response corresponding to data is retrieved from the Service's storage
• parts of response is sent through the network (or we do some rmc stuff with it)

Heavy parts:

• refactor Handler so that there is no on_message(message) but it's split into a number of functions dependent on which variant is the message
• refactor Service so that match message happens inside handle_network_alert and not in on_message in Handler (which no longer exists).

This is done so that backup save can be triggered inside Service and not handler (it needs to happen only in a specific variant of message).

[github-actions]

Please make sure the following happened

• Appropriate tests created
• Infrastructure updated accordingly
• Updated existing documentation
• New documentation created
• Version bumped if breaking changes

[timorl]

Nice work, only two tiny comments, you might even want to just ignore the latter if it's too much work.

[kostekIV]

One comment, possibly ignorable