Support pre-registered schemas

It’d be great to have support for the schema-check, rather than only schema-register functionality of getting an id for a given schema in the schema registry. From the docs:

# Check whether a schema has been registered under subject "Kafka-key"
$ curl -X POST -i -H "Content-Type: application/vnd.schemaregistry.v1+json" \
    --data '{"schema": "{\"type\": \"string\"}"}' \
    http://localhost:8081/subjects/Kafka-key

Rationale: We want to do the register outside of server invocation (eg, in the CI deployment, before the actual code deploy), so that if a developer is working with a local branch against a shared dev environment, they won’t accidentally register a new schema by launching the server on a WIP branch.

[Nevon]

This actually exists, and just isn’t documented:

https://github.com/kafkajs/confluent-schema-registry/blob/master/src/SchemaRegistry.ts#L135-L147

[jbooth-mastery]

LGTM! Better than I would have done too, I only dabble in typescript.