Add MySQL to CI workflow

[karlwilbur]

This PR adds MySQL to the CI workflow, as mentioned in #1369.

There is currently a MySQL syntax error in the 0.10.x code that would be detected in CI with MySQL in the testing matrix.

All Submissions:

• I've checked to ensure there aren't other open Pull Requests for the same update/change.
• I've submitted a ticket for my issue if one did not already exist.
• My submission passes all tests. (Please run the full test suite locally to cut down on noise from travis failures.)
• I've used Github auto-closing keywords in the commit message or the description.
• I've added/updated tests for this change.

New Feature Submissions:

• I've submitted an issue that describes this feature, and received the go ahead from the maintainers.
• My submission includes new tests.
• My submission maintains compliance with JSON:API.

Bug fixes and Changes to Core Features:

• I've included an explanation of what the changes do and why I'd like you to include them.
• I've provided test(s) that fails without the change.

Test Plan:

Reviewer Checklist:

• Maintains compliance with JSON:API
• Adequate test coverage exists to prevent regressions

[karlwilbur]

Link to GH Actions for my fork:
https://github.com/K-and-R/jsonapi-resources/runs/2698784105?check_suite_focus=true

[bf4]

Looks like tests started failing after merge if you can look https://github.com/cerebris/jsonapi-resources/actions/runs/3932254461/jobs/6724583500