Feature: enable support for alternative schema stores

[markdav]

The schema registry uses kafka as the schema store, which makes sense as it's geared towards environments with Kafka in-situ. However it seems that versioning of avro schemas as a service is not fundamentally related to kafka but is a problem space in its own right. It might be useful to be able to configure the schema registry against alternative stores in the following situations:

• for sites where alternative stores are already HA but Kafka is not
• for sites where kafka is not part of the infrastructure - schema registry could still be used for versioning of avro messages stored in alternative locations (file store, hadoop, etc).
• for sites where kafka is running in secure mode (which schema registry doesn't support)

Stores which might be useful to add might include

• relational RDBMS
• non-relational e.g. hbase, cassandra, elastic.
• even file-base storage

[ewencp]

@markdav I think the necessary interface is already there -- io.confluent.kafka.schemaregistry.storage.Store is the interface, KafkaStore is the only "real" implementation currently, and InMemoryStore is used both in KafkaStore's implementation and for testing. I think the biggest issues would simply be around configuration and then how we'd handle additional implementations (given the number of options, could be in separate repos and you just have to get the jars on the classpath to instantiate the desired store type). In the core schema registry code, making this pluggable is probably actually a pretty small configuration-focused patch.