Add GET endpoint for controlled keyword options (keys) #623
Description
We have a GET endpoint for getting the possible values a keyword can have, but do not have an endpoint for getting keyword keys a user can submit. This means that to submit controlled keywords via the API, a user would need advance knowledge of what keywords we accept.
We should add an endpoint which provides them a list of such keywords.
As part of this, we should consider whether making the keywords and their values enumerated properties would be worthwhile. Such a change would give us Pydantic validation upfront of keywords and values, but would also make keyword changes require internal code changes. I'm inclined to think this would be a worthwhile change, since (1) the controlled keywords are pretty static and (2) changes to them aren't time sensitive.