Add a button (or a background task) to identify the latest received shielded UTXOs when using a remote node

[dbadol]

Current situation
As explained in #1123 using a remote node will not automatically identify changes in the user's shielded UTXOs. Only a full (and long) rescan would allow identifying the latest received offline transactions.

Solution we propose
The latest received offline transactions could be identified by a quick rescan that would only scan the shielded UTXOs and only from a given block height.

This specific mini-rescan could be done seamlessly at wallet start, automatically and incrementally (since the last rescanned block height).

Doing it at wallet start is a rather intuitive solution that doesn't clutter the GUI. From the user's point of view, if they are using a remote node, they'll need to close and reopen the wallet to see any newly received offline transaction.

Alternatives considered
An alternative would be to allow the user to launch such mini-rescan through a refresh button (or a settings options).

Additional context
This issue implies developing such incremental mini-rescan in the nodes, which @valdok says it can be done. Issue entered in BeamMW/beam#1973.