add 'support_versions' endpoint

[hunterowens]

To each API, add a supported version for programmatic checking of supported versions. Related to #147

[hunterowens]

How to articulate this, IMO.

MDS, as a spec, supports 2 minor versions. (ie, 0.2 and 0.1 rn)

Cities, as part of their regulatory guidance, can require particular version #s. For example, SM may require latest while LA requires latest or latest -1.

Finally, implementations, of either provider or agency, support n versions. For example, company_x’s Provider implementation might support 0.1.1, 0.2.0 and 0.3.1 while LA’s agency implementation might support 0.3.0

[oderby]

@hunterowens after writing up #216, I'm not sure I see the reason/value in this. Can you elaborate on what solving this would enable?

[hunterowens]

yeah, with #216, I feel like this might not be required.