Establish a standardized catalog config specification and corresponding validation tests

[yymao]

During the early development stage of GCRCatalogs, one of the guiding principles was to give as much flexibility as possible to the catalog providers, to lower the barrier for catalog providers who want to make their product accessible via GCRCatalogs.

With GCRCatalogs now being one of the main data access methods for a wide range of different data products in the DESC, the flexibility that we gave to catalog providers has gradually turned into maintenance costs. As of today there are 29 readers and more than 130 catalogs.

One step to reduce future maintenance cost is to make establish a standardized catalog config specification.

In fact, several recent updates are already moving toward that direction. In particular we have established a common root_dir for all the paths that appear in catalog configs, and have also established a few "reserved" config keywords. So we are indeed getting closer to having a more uniform, formally-defined catalog config structure.

There's still work, however. For example, we would need to homogenize different keywords used to define file/catalog paths. We may want to have a more structured content for deprecated key. We may want to homogenize the way people specify a subset of catalog files (for example, right now object catalogs use regular expression while cosmoDC2 uses a list of integers).

This issue supersedes #345.

[yymao]

Equally important are the tests to validate catalog configs, ensuring that they do follow the specification.

There have been some discussion between @JoanneBogart and me in #446. We have now already established a few reserved keywords (and are documenting them inCONTRIBUTING.md, see #446), so in principle we can already add tests to validate catalog configs that include any of these reserved keywords.

That said, the design of a specification and the tests to validate it are of course highly related. So it would be good to think about these two aspects together when working on the design.