feat: add functions to help create sighashes

[djordon]

Description

Closes #804.

Changes

• Add a new table that is similar to some of our sweep table.
• Write to this new table when we write sweep transactions and donations.
• Update the get_signer_utxo function to use a new sweep table. That's most of these changes.
• Add function that validates an individual deposit.

Testing Information

This needs an integration test for the block observer.

Edit: This code was changed in #822, and I add an integration test there.

Checklist:

• I have performed a self-review of my code

[cylewitruk]

I know you're not done but here's my comments so far