feat: Simplify FlagsmithCache interface

[rolodato]

The FlagsmithCache interface is unnecessarily complicated:

• has is never used by the SDK, it is only used in tests. This change removes it.
• The return value of set is never used by the SDK, and serves no purpose. This changes set to be of type Promise<void>.
• set accepts a ttl parameter but the SDK never actually uses it. This change removes it.
• The return type of get used to be Promise<Flags | undefined> | undefined. In practice, the SDK always awaits this value so it should be Promise<Flags | undefined>, which is what this change uses.

The Flagsmith constructor also checks that the supplied cache has the same methods as the ones required by FlagsmithCache. This check is not thorough enough to be useful and should be done by typechecking. Also, this checking requires FlagsmithCache to be indexable by a string, which customers will never need to do and just adds boilerplate. This change removes this check and should be enforced by typechecking.

This change also adds JSDoc to FlagsmithCache, so we can link to it directly from docs and avoid duplicating code snippets or explanations.

These are technically breaking changes, but only for TypeScript users and has no effect at runtime. Prior to 4.0.0 it was impossible to actually refer to the FlagsmithCache in TypeScript code because we were not exporting any types. I'd be okay with releasing it as 4.1.0.

[matthewelwell]

Looks fine, but I'd rather release a major version here. I don't see any reason not to?

[rolodato]

Sure, we can release it as new major version.

[matthewelwell]

@rolodato ok, can you merge and handle the release?