[electrum] define and annotate Interface attributes in __init__.py

Summary:
Monkey-patching the Interface instances in network.py with new attributes makes it hard to inspect the code and know what the attributes are. Declare them in Interface.__init__ and initialize the ones that were previously initialized immediately after instantiating the Interface

Depends on D14950

Test Plan:

python test_runner.py
pytest electrumabc/tests/regtest

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14951