Should there be a KeySerializer?

There's `ValueSerializer`, `ValueDeserializer`, and `KeyDeserializer` but no `KeySerializer`. Should there be one?

I'm on Spring Boot and trying to convert my modules into `@JacksonComponent`. I noticed that the annotation won't work for key serializers, since there is no base class for them, as they're just regular `ValueSerializer`s that use `writeFieldName`.

Start by comparing the existing ValueSerializer, ValueDeserializer, and KeyDeserializer abstractions and reviewing how @JacksonComponent discovers components. Determine whether key serializers need a distinct base class for annotation support; done requires a documented design decision and agreed scope for any follow-up work.

Written by the indexing model from the issue text.