Clean up the schemas and make them compatible with a code generator

[WilliamJamieson]

This PR addresses some major clean-ups of the RAD schemas as well as some minor changes to them in order to support
automatic python code generation from them.

Highlighted changes:

• The rad_schema-1.0.0, which is the meta-schema for rad, has been moved out of the schemas directory into its own directory. This is because its not really a "schema" is a schema for the schemas and so it confuses the generator.
• The tagged_scalars have been moved back into basic-1.0.0 directly. This is effectively reverting RAD-71 Move archive information to be in tags #153. The code generator has methods to overcome the limitations that caused this change.
• All $ref have been updated to use absolute references via the schema uri specified for each schema (its id) instead of the mix of relative references. Having a single-consistent reference system makes things much simpler.
• All data_product (top-level non-reference file data model schemas) have been moved into their own directory. This makes the meaning of these schemas much clearer.
• Move ref_common, ref_exposure_type, and ref_optical_element out of reference_files into the main directory. This is so that reference_files only contains top-level schemas describing the reference files.

Checklist

• Schema changes discussed at RAD Review Board meeting
• Added entry in CHANGES.rst under the corresponding subsection
• Updated relevant roman_datamodels utilities and tests
• Passed romancal regression testing on Jenkins / PLWishMaster. Link: https://plwishmaster.stsci.edu:8081/job/RT/job/romancal/XXX/

[codecov]

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