Fully abstract XContentType to MediaType

[nknize]

Is your feature request related to a problem? Please describe.

I just realized we didn't have an explicit issue for this. XContentType is used to specify the string format for several different mechanisms (e.g., REST I/O, ClusterMetadata, any of the 1379 class that that implement toXContent). Today the core support four options (JSON, YAML, SMILE, CBOR) as defined in this class. These concrete implementations exist in the :libs:opensearch-xcontent library such that downstreams could (theoretically) implement their own XContent format. However, XContentType and JSON is hard coded in various places throughout the code base.

Describe the solution you'd like
As part of enabling REST API Versioning, I've started abstracting the core to MediaType such that XContentType only defines the concrete implementations provided by the core xcontent library, and MediaType provides the interfaces provided by the core library.

This meta issue is to see that refactor to completion to fully decouple plugins, modules, and :server from the core XContentType implementation by switching over to the MediaType base interface.

Describe alternatives you've considered
Leave it tightly coupled.

relates #5910
relates #8110