Schema Registry Client: Does not support Context operations

[alexhein]

This is a gap in the public methods provided by the schema registry client w.r.t. the REST API of Confluent Schema Registry.

In particular, it is not supported to list contexts.
See API Reference: https://docs.confluent.io/platform/current/schema-registry/develop/api.html#get--contexts

In addition, the client does not allow to specify a base context path as described in https://docs.confluent.io/platform/current/schema-registry/schema-linking-cp.html#using-a-base-context-path
to realize requests of the form GET <urls>/contexts/<context-id>/schemas/ids/<id>/versions

Golang Interface of the client: https://github.com/confluentinc/confluent-kafka-go/blob/v2.3.0/schemaregistry/schemaregistry_client.go#L199