wallet: target a pre-defined utxo set composition by adjusting change outputs

**Describe the enhancement**

This is an interesting feature that was suggested for the bitcoin core wallet that I think could be useful for bdk wallet users also. See description here: https://github.com/bitcoin/bitcoin/pull/29442

**Use case**

This would be useful for LN liquidity providers who are using BDK instead of the core wallet, or any other users who might want to maintain a utxo set composition of pre-defined sizes. Could this be useful for payjoin or coinjoin?

**Additional context**

This is very low priority right now, but putting it out there as a reminder for future discussion.