[chronik] Electrum: implicitely enable the scripthashindex if the electrum interface is used

Summary:
This is a better UX. It still errors if the index is explicitely disabled.

Depends on D18112.

Test Plan:

./test/functional/test_runner.py chronik_electrum_*

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D18113