Add support for custom comment message

[baierjan]

Feature is used by openSUSE/openqa-tests-backlog#4

Reference: https://progress.opensuse.org/issues/116605

[baierjan]

The failing check seems irrelevant

Run marocchino/sticky-pull-request-comment@v2
Error: Resource not accessible by integration

And it also seems that the unit test is not running at all in the pipeline; so at least locally:

~/Projects/backlogger$  pytest
================================= test session starts ==================================
platform linux -- Python 3.10.6, pytest-7.1.2, pluggy-1.0.0
rootdir: /home/jbaier/Projects/backlogger
plugins: requests-mock-1.9.3, mock-3.6.1, cov-3.0.0
collected 1 item

tests/test_comments.py .                                                         [100%]

================================== 1 passed in 0.05s ===================================