[CI] Add cashtab coverage report to the coverage panel

Summary:
This diff makes the coverage report panel generic, so that any project
with a coverage summary artifact will be added automatically. This
effectively adds CashTab coverage report to the panel after D8874 is landed.

Test Plan:

pytest

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8875