Normalize avro union format for the type while registering the schema

[shiveshr]

Problem description
Union type Avro schemas can be used to express complex schemas where the type is supplied for the schema.
This is fine generally, but when we normalize, we should normalize it as the type specific schema as a nested structure.

Problem location
SchemaRegistryService

Suggestions for an improvement
Normalize the union into a top level schema for the type.