Expose JSON Schema ID and maturity as class methods

[korikuzma]

Feature description

In the spec:

{
  "$id": "https://w3id.org/ga4gh/schema/cat-vrs/1.0.0/json/CanonicalAllele",
  "maturity": "trial use"
}

I imagine others will want this too. It'd be useful to expose this info through class methods, like the following:

CanonicalAllele.schema_id()
# "https://w3id.org/ga4gh/schema/cat-vrs/1.0.0/json/CanonicalAllele"

CanonicalAllele.maturity()
# "trial use"

It'd also be nice to expose this metadata in model_json_schema

Use case

As I have began working on the starter kit and bundle format for civicpy, I need a way to resolve the schema identifiers.

Proposed solution

Follow similar approach as ga4gh/vrs-python#650