Support `@JsonSerialize(keyUsing)` and `@JsonDeserialize(keyUsing)` on Key class

[cowtowncoder]

(note: follow up on #2454)
Although keyUsing property of @JsonSerialize and @JsonDeserialize annotations is supported for properties (added to Methods and Fields) -- as well as when added on Map (sub-)class, one specific usage that seems intuitive is not yet supported as of 2.10.0:

@JsonDeserialize(keyUsing = MyKeyDeserializer.class)
@JsonSerialize(keyUsing = MyKeySerializer.class)
public class KeyType {
    // ...
}

But it probably should be.

[cowtowncoder]

Implemented for 2.11, will be in 2.11.0 (which will take a while, pre-releases by end of 2019).