Initial draft of enrollments API

[korylprince]

This is an initial draft for an enrollments API (see #6).

So far this just includes a storage interface and OpenAPI spec - once these are finalized/agreed upon, I'll work on the implementation.

This API provides just enough query and response fields to be useful, and specifically avoids needing to reparse raw Authenticate, UserAuthenticate, or TokenUpdate bodies in the data store.

[jessepeterson]

This is great! Thanks for looking at this! I only have time at the moment to review this specific part (the query input). I'll hopefully get more time to look and think about the others soon!

[korylprince]

@jessepeterson updated everything based on your suggested changes.

[jessepeterson]

Moving in the right direction, I think. In general I think I would prefix all structs with EnrollmentsQuery* to solidify all these various structs are part of the same "suite" (save for Pagination — that can be re-used between any other queries).

Encoding the error in an actual HTTP response I'm not sure if I'd wrap the whole result in another structure that embeds the EnrollmentsQueryResult to capture and pass on the error as well as any results. The semantics are different for a REST endpoint vs. using e.g. Go library code.