Switch to using WarpStream-compatible schema registry routes

[jlrobins]

the GET /schemas route isn't implemented in WarpStream schema registry. Transition away from it in favor of two different layers of route calls and cached data.

Layer one:

1. List subjects. Returns list of strings of subjects that have registered schemas. Cache this list of strings related to the schema registry id. This would be sufficient to use for topic characterization of being schema'd or not, as well as the toplevel view of contents of a schema registry, albeit w/o knowing the schema type or number of versions within.

Layer two:

1. List versions under subject, then returning list of live version numbers for a single subject.

2. Then with each subject name + version number pairing, loop over Get schema version given subject name, GET /subjects/{subject}/versions/{version} . Will return all of the binding metadata as well as the schema document. Gather up the interesting bits other than the schema document and cache this list of Schema object related to {schema registry id, subject} key. This is essentially the schema subject group construct we have in the codebase already.