Move APV to Core and remove shader config dependency + auto load in player without external script

[FrancescoC-unity]

This PR does a couple of things:

• Remove the dependency on shader config. Substituted it with a multi_compile in shader.
• Added a multi compile for L1 and L2, at the moment L2 doesn't do anything different, it is just a setup for when we'll implement.
• Move practically all files to core.
• Added warning in editors if anything but HDRP is used.
• Changed visibility for several things.
• Added some documentation for stuff that is public.
• Made it so the asset gets loaded directly from the component that is used in editor. Previously the system needed an external per-project script, now is not needed.
• Deleted some code that was dead.

[thefranke]

Nice work, thanks!