assets swap deposits [2/N]: extend the loop tapd client with funding, signing, and swap support utilities

[bhandras]

This commit adds methods to the tapd client for preparing, funding, signing, and publishing virtual PSBTs, managing asset balances, handling proofs, and watching for asset send/receive completion events. Also introduces utility methods for creating OP_TRUE sweep vpackets from asset proofs.

Build on top of: https://github.com/lightninglabs/loop/pull/979

Pull Request Checklist

• [ ] Update release_notes.md if your PR contains major features, breaking changes or bugfixes