Add a `@JsonComponent` annotation that can be used to indicate that a
Bean is a JsonSerializer and/or JsonDeserializer that should be
registered with Jackson.

Support is provide via a new `JsonComponentModule` which is
auto-configured in `JacksonAutoConfiguration`.

Also add `JsonObjectSerializer` and `JsonObjectDeserializer` classes
which provided as a convenient base for serializers that work with
Objects.

Fixes gh-3898