Light client improvements

[gregdhill]

• [ ] Implement metrics getters (expensive - ignore?)
• [x] Implement bump fee logic
• [ ] Use wiremock for testing electrs
• [ ] ~~Support signing other input types (currently only p2wpkh)~~
• [x] Separate funding logic from estimation (see AvailableCoins)
• [x] Calculate minimum change amount
• [ ] test/improve error handling
  • [ ] Improve is_transport_error such that the light client networks are caught e.g. here.
  • [ ] look into how we handle ElectrsError::ReqwestError. In the case of rescan_electrs_for_addresses it's probably fine not to add special handling for it, and just to treat it as a critical error.
  • [ ] Check that light client errors are handled correctly in general

Related:

• #464