This will make the next refactor slightly easier.

Signed-off-by: Ben Pope <ben@redpanda.com>

This will make the next refactor slightly easier to comprehend.

Signed-off-by: Ben Pope <ben@redpanda.com>

Signed-off-by: Ben Pope <ben@redpanda.com>

This should be a pure refactoring.

The store has two mappings:
1) `schema_map`: Schema ID to Schema (sharded across cores)
2) `subject_map`: Subject-Versions to Schema ID

Each of these mappings are then split across shards, coordinated
by sharded_store.

Prior to this change, references were attached to the Subject-Versions.

This is an incorrect modelling; Schema should own the references
to other schema. This commit does that refactoring by moving references
from `typed_schema` to `typed_schema_definition`.

There are a couple of things that are not straightforward:
1) `ValidSchema` (`protobuf_schema_definition`, `avro_schema_definition`)
   are the respective library instantiations of the representation,
   and are unable to reproduce their references to form a
   `canonical_schema_definition`, so references are stored inside the
   wrappers.
2) `referenced_by` and `is_reference` now require two phases to
   obtain the references, due to the sharded and diconnected nature of
   the two mappings (schema, and subject-versions).
   a) Obtain a list of schema ids that reference the subject-version
   b) Filter that list by subject-versions that are not soft-deleted

Signed-off-by: Ben Pope <ben@redpanda.com>

Signed-off-by: Ben Pope <ben@redpanda.com>

`GET /schemas/ids/<id>` now returns its references.

Fixes redpanda-data#11194

Signed-off-by: Ben Pope <ben@redpanda.com>