fix(internal schema): changes to make internal schema registry more compatible with other schema registries

[darnaut]

• Implementation for the Schema Registry APIs /subjects/{subject}/versions/{version} (getSchemaByVersion) and /subjects/{subject}/versions ( listVersions) which are used by Apache Gobblin to get the latest version of a registered schema. Since schema versions are always 1, these methods are rather straightforward. getSchemaByVersion with version 1 or latest returns the schema registered for a requested subject, otherwise it returns not found. listVersions returns version 1 if a schema registered with the requested subject exists, otherwise it returns not found.
• Since schemas are not registered with a schemaType, this field is not currently returned when an external Schema Registry is used and is thus removed from the response for compatibility. Additionally, the maxId field should only be returned when requested.

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