[electrum] implement TxInput.to_coin_dict and TxInput.is_complete

Summary:
Move the serialization to the legacy dict format into TxInput

This opens the way to using TxInput internally in the Transaction class while still being able to convert to the legacy dict where it is really needed (saving coins to JSON)

Add tests related to input completeness (incomplete inputs have a value and a xpub)
Depends on D14459

Test Plan: python test_runner.py

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14461