Add endpoint for specific schema version

[torbsto]

This PR adds support for fetching the version for a specific schema. It adds a handler for POST requests matching /subjects/[^/]+\\?deleted=true , which is called by schemaRegistryClient.getVersion(topic, schema) . The AvroConverter uses this method internally.

[philipp94831]

Why is deleted=true part of the pattern?

[torbsto]

The deleted parameter is set to true when calling getVersion(subject, schema) and to false when calling getId(subject, schema).
Originally, I did not plan to include support for id requests and I wanted to make sure that there are no unexpected responses. Especially, since the parameter is not documented anywhere.
I looked into the behavior of a real schema registry and it responds as expected: When set to false, querying a deleted schema (not subject!) returns `{"error_code": 40403, "message": "Schema not found"}. Otherwise, it returns the complete schema information.
I'll remove the parameter from the pattern and add the logic for both cases in the transformer.

[torbsto]

The SR Client Mock doesn't have any information about the version of a deleted schema. Therefore, I suggest that we treat a deleted schema as if it does not exists. This should be sufficient for testing applications.