Create optional build for user-contributed code

[AlysonStahl-NOAA]

Part of #206

This PR sets up the build structure for user-contributed code in the "extra" subdirectory, starting with code that enables MySQL compatibility.

[AlexanderRichert-NOAA]

Is the idea that users will contribute code that lives in the "extra/" directory? If so then I don't know if it should necessarily get tested in CI, depending on how much work you foresee that being to maintain.

[AlysonStahl-NOAA]

Is the idea that users will contribute code that lives in the "extra/" directory? If so then I don't know if it should necessarily get tested in CI, depending on how much work you foresee that being to maintain.

Yes and the idea the user contributed code won't be tested. The test script I set up just checks that the functions are added to function list if the build option is turned on, but the code itself isn't tested.

I mostly created the test for myself while setting this up, but I wanted a test that would break if contributors don't follow the necessary steps. However, they would also need to update the test as well and we can look for this in code reviews anyways.

I'll just remove the tests.