[electrum] make parse_input return a TxInput

Summary:
This is part of the effort to progressively move the coin dicts from low level code to higer-level code, to eventually only use it when exporting coins to JSON.

Depends on D14470

Test Plan: python test_runner.py

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14471