Add Kafka event return support

[zmingee]

What does this PR do?

Add event return support to Kafka returner module.

Also added a test module and refactored the module.

Previous Behavior

Event returns were not supported by the Kafka returner module.

New Behavior

Event returns are now supported by the Kafka returner module.

Merge requirements satisfied?

[NOTICE] Bug fixes or features added to Salt require tests.

• [Y] Docs
• [N] Changelog - https://docs.saltstack.com/en/master/topics/development/changelog.html
• [Y] Tests written/updated

Commits signed with GPG?

Yes

[krionbsd]

@zmingee could you move the test under tests/pytests/unit/returners and create changelog entry?

[zmingee]

@krionbsd I've moved the test module and added the changelog entry as requested.

[whytewolf]

@zmingee There seems to be some problems with this PR, some items are being pulled in from other PRs from the last time you pulled in from master. and there are conflicts. Can you please correct these issues.

[dwoz]

Closing this due to inactivity. Anyone should feel free to re-open it if they want to see it through to the end in one release cycle.