[CHORE] Define the versioning relationship between flagd daemon and providers

We need to officially document the relationship between the "flagd daemon's" version, and the version of any provider (even if there isn't one).

Comment from @toddbaert:

I think I'd still like to do this with the flagd schema, and otherwise keep this versions uncoupled.

Denoting compatibility via the schema is one means of doing this. Though it won't cover everything, it can at least cover the functionality of operators... IE if we added a new operator, we could release a schema version 1.X, and providers not supporting this operator would warn or error if they were compatible with flagd schema version < 1.X.

It's likely impractical to keep versions in sync since breaking changes in a provider implementation might occur completely independently of changes in other providers, or the flagd daemon. We should document whatever decision is made though.

The issue names no file, test, or entry point; begin by resolving the proposed relationship between the flagd daemon, providers, and the flagd schema. Done means a documented versioning policy that states compatibility expectations, including the no-provider case, and the limits of schema-based compatibility.