Support `protobuf` model cross-document `protobuf schema` references

[jfallows]

Describe the desired outcome from the user's perspective As a catalog schema author, I want to be able to use cross-document protobuf schema references when supported by the catalog.

Acceptance criteria

• Resolve and use cross document references in protobuf model validator and converter

Additional context Check limitations of cross-document reference support by various catalogs, such as karapace and apicurio.