Introduce module for compat code

[jakelandis]

@pgomulka

I am reviewing elastic#53228 and am a bit concerned with how spread out the compat changes are. I would like to suggest trying to encapsulate as much as possible in oss/modules or x-pack/plugin. I got an oss/module started and moved the RestGetAction for the compatibility layer over.

If you agree with this, could you accept this PR (which will update elastic#53228). Then can you move as much of code as possible over to it over to the module ? Ideally 100% of the compat code would be in the plugin, but that is not realistic.

We should be able to move all of the RestHandlers pretty easily (as demonstrated in this PR).
I think the scaffolding/core parts are O.K. to live where ever it makes the most sense, but we should strive (if possible) to encapsulate as much of it in the oss/module even if it means some SPI indirection.
Any code specific to a version should be in the oss/module.
I understand the x-content code (and eventually the transport code) isn't so easy to encapsulate, so in the meantime we can just leave them where they are. I like how they mirror the transport code, but would like it better if the module could contain the business logic applied. I have a couple ideas floating around for how we could do that, but none are very good or well formed (callback ?, x-content fragment registry ?, serialize->deserialize->mutate->serialize) . I will hack on those ideas some soon, or happy to hear any ideas you may have to encapsulate the x-content version specific code.

Included in this PR:

• synced up with the compat_rest_api branch (sorry for the noise)
• new oss/modules - with spotless enabled
• moved the RestGetAction compatiblity to the module
• removed dead? loggers in the real RestGetAction
• fixes to allow PreCommit to run sucessfully

[pgomulka]

looks good!!