Introduce @CodecProvider for discovery of user supplied CodecProviders

[evanchooly]

see this discussion for context.

[eheimburg]

I've been thinking about solutions to the enum-codec bug (#2264) that doesn't impact performance. (The obvious fix is to remove the built-in enum-to-string Converter, but I'm not sure about the performance impact.)

Maybe this proposed new @CodecProvider annotation could help here too: maybe the built-in converter could first check if that annotation is present for the enum -- if it is, it can just not convert that type.

[evanchooly]

this will be better handled with 3.0's build-first approach