[Chronik] Use ffi client_name instead of hard-coded string

Summary:
Currently, "Bitcoin ABC" is hard-coded in electrum.rs.

However, it is better to get the client name from clientversion.cpp. This way, forks of Bitcoin ABC with a different name (*cough* doged *cough*) don't have to update this hard-coded string.

Test Plan: ninja && python ./test/functional/test_runner.py chronik_electrum_basic.py

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D18657