Support schema-registry "subjects", not only schema IDs

[synhershko]

Assuming forward compatibility of schemas, the REST proxy should accept accepting messages into Kafka topics with a given schema "name" and not just an explicit ID

[ilkkao]

I was also surprised to find out that it's not possible to give subject name to use the latest schema under that subject. Also the docs are not that clear if any schema id is supported or only those schemas that have been inserted via previous REST request:

key_schema_id (int) – ID returned by a previous request using the same schema. This ID corresponds to the ID of the schema in the registry.

I assume any schema id existing in registry can be given.

[ilkkao]

Same question raised in https://github.com/confluentinc/kafka-rest/issues/424 No real answer there either. The proposed two step workaround adds a lot of overhead. Especially if there's no caching and it's done for every produced message separately.