[chronik] Electrum: don't enforce returning an array in notifications

Summary: This prevents the array from having several inside content, so better build directly the array at callsite. Also it makes it possible to return named parameters instead of positional parameters if needed in the future. For now there is no change in behavior.

Test Plan:

./test/functional/test_runner.py chronik_electrum_*

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D18105