bark: Add new VTXO input selection logic

• check or remove this line after you've added a CHANGELOG entry or if your PR doesn't need one (they should go into CHANGELOG/unreleased/)

This is a prerequisite for !2085 (merged) and it will also help with improving our payment logic so we can exclude VTXOs from input selection if the server reports unusable inputs to us. I made the method public too so other devs can use it where desired.

Summary by CodeRabbit

• New Features

  • Added more flexible VTXO input selection across sends, lightning payments, receive flows, fee estimation, and on-chain withdrawals.
  • Introduced controllable input capping and exclusion rules to better manage how many inputs are used and which ones are preferred.

• Bug Fixes

  • Improved consistency in how funds and fees are estimated by using the same refined selection approach across payment paths.
  • Updated receive and fee-estimation behaviors to reduce unexpected selection differences, including better handling under the “no explicit token” case.