Light client improvements

[gregdhill]

• Implement metrics getters (expensive - ignore?)
• Implement bump fee logic
• Use wiremock for testing electrs
• Support signing other input types (currently only p2wpkh)
• Separate funding logic from estimation (see AvailableCoins)
• 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:

• Use bdk as a dependency for signing #464