Add support for schema pluggability and schema references

[rayokota]

This PR add supports for pluggability and schema references.

The notion of a schema has been abstracted into a new interface in Schema Registry called ParsedSchema. Implementing a ParsedSchema and a SchemaProvider are all that is needed to add new a schema type to Schema Registry.

In order to add further schema types, the following property should be set in the Schema Registry properties file:

schema.providers=my.package.MySchemaProvider

Avro, JSON Schema, and Protobuf all have the notion of schema references (also sometimes referred to as schema imports or schema includes). This feature is less important for Avro since the feature is associated with their protocol functionality, which is not used by Schema Registry. However, for JSON Schema and especially Protobuf, the use of schema references is more prevalent. Therefore Schema Registry now supports the notion of schema references. A schema reference is comprised of

A name for the reference.

For Avro, it would be a fully qualified schema name.
For JSON Schema, it would be a URL.
For Protobuf, it would be the name of another Protobuf file.
A subject, representing the subject under which the referenced schema is registered.
A version, representing the exact version of the schema under the registered subject.
When registering a schema, the associated references, if any, need to be passed. Typically the referenced schemas would be first registered, then their subjects and versions can be used when registering the schema that references them.

When a schema that has references is retrieved from Schema Registry, the referenced schemas will also be retrieved if needed.

[tuvtran]

Thanks! LGTM!