feat: make signal argument optional

[rgraber]

Description:
Make the 'signal' argument to the consumer optional in preparation for deprecation. Remove checking against the passed signal since more than one signal type may come from the same topic. See https://openedx-events.readthedocs.io/en/latest/decisions/0010-multiple-event-types-per-topic.html.

GH: openedx/openedx-events#218

Reviewers:

• tag reviewer
• tag reviewer

Merge checklist:

• All reviewers approved
• CI build is green
• Version bumped
• Changelog record added
• Documentation updated (not only docstrings)
• Commits are squashed

Post merge:

• Create a tag
• Check new version is pushed to PyPI after tag-triggered build is
  finished.
• Delete working branch (if not needed anymore)

Author concerns: List any concerns about this PR - inelegant
solutions, hacks, quick-and-dirty implementations, concerns about
migrations, etc.

[bmtcril]

Thanks for doing this!