feat: implement get_signer_utxo for in memory storage

[matteojug]

Description

Initial work for #538.

I'll add the postgres impl (and more tests, if needed) in a different PR to keep this small, if there are no objections :)

Changes

• Move get_signer_utxo to storage trait (as the issue suggest to use the database)
• Implement get_signer_utxo for in memory db
• Add some tests for it

Testing Information

Since we no longer mock it, should_be_able_to_coordinate_signing_rounds now also need sbtc txns to work.

Checklist:

• I have performed a self-review of my code
• My changes generate no new warnings
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[djordon]

Nice. Gave it a quick look over just now and will have a proper review tomorrow. Here is what I have so far.

[cylewitruk]

Tossed in a few comments/suggestions. I didn't look too hard at the tests as I think there will be a bit of refactoring there after this comment. Good work so far!

[djordon]

Looks good.

I think the logic looks sound. But can we add another test for the case where we have a fork with chains of the same length and UTXOs on both? You can either modify an existing test or add a new one.

[djordon]

Looks good ✅