feat(wallet): add functions to lock and unlock utxos

[notmandatory]

Description

fixes #849

Add functions to Wallet to lock and unlock utxos. The locked TxOuts are automatically added to TxBuilder's "unspendable" list. This is just a draft to see if this approach makes sense, if so then I'll add persisting the locked utxos.

TODO

• add locked_unspent and unlocked_unspent to Wallet::Changeset
• check the Wallet.locked_unspent list after Wallet::apply_update and remove locked spent UTXOs
• update store crates to persist new locked_unspent data

Notes to the reviewers

The design is based on how the core wallet locks UTXOs except I think it's simpler to always persist locked utxos. See: https://bitcoincore.org/en/doc/28.0.0/rpc/wallet/lockunspent/

Should locked UTXOs that are found to be spent be removed from the locked list after only one confirmation?

If API looks good then it won't be a breaking change so I propose this PR goes into the 1.1 milestone.

Changelog notice

• Add wallet functions to lock and unlock UTXOs. The list of locked UTXOs is persisted with other Wallet data.

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

[tnull]

Should locked UTXOs that are found to be spent be removed from the locked list after only one confirmation?

I think that would be reasonable, although allowing for a larger reorg threshold (usually 6 conf) might be safer?. And, while having this explicit API available is great, as previouslydiscussed, it would be great if BDK could offer a way to specify to auto-unlock the UTXOs after some (block or time) delay. Otherwise users would still need to track these UTXOs independently/outside of the Wallet and remember when to unlock them.

[notmandatory]

@tnull for unlocking how would you feel about having explicit functions, something like this:

1. add Wallet::lock_unspent_until_height(outpoint: OutPoint, block_height: u32) -> bool which would lock until the given block height.
2. add Wallet::update_locked() -> Vec<OutPoint> which would unlock locked UTXO if they have 6 confirmations or their given unlock height was reached. Would return list of unlocked UTXOs.