Modify the schema to apply callbacks or required parameters to nested objects

[rachelbaker]

Allow schema properties like sanitization_callback, type, required, etc. to be set and processed on nested objects.

Currently rest_sanitize_request_arg() and rest_validate_request_arg() only handle properties set on the top level of nested objects, which means if we try to set properties on content.raw they are ignored.

[rachelbaker]

Split off from #2758

[rachelbaker]

@rmccue You mentioned that you might take a swing at this for Meta, still interested?