[electrum_redesign] Introduce redesigned `ElectrumExt`

[electrum_redesign] Introduce redesigned `ElectrumExt`

There are a number of improvements that can be done, but it is in a
decent state to be usable.

Possible improvements:

* Remove requirement to retry obtaining ALL data after reorg is
  detected. Transactions can be anchored to a lower block (not block
  tip), and an `assume_final_depth` value can be used.

* The logic to finalize an update with confirmation time can be improved
  during reorgs to not require returning an error.