Support using direct connections in Kafka REST, Message Consumer, and SR API endpoints

[rhauch]

Continuation of #96, and followup to #116 that added initial and limited direct connections (with GraphQL support) but did not register the kafka or SR cluster in the cluster cache, and did not verify the Kafka REST (including new produce support), Message Consumer and SR APIs work as expected.

Note that direct connections do not yet support credentials (see #116), so direct connections will only work with Kafka clusters and SR that have no authentication. We'll track adding support for various credential types in other issues:

• basic authN (Add basic credentials/authN to direct connections #124)
• mTLS authN (Add mTLS authN to direct connections #125)
• OAuth authN (Add OAuth 2.0 authN to direct connections #126)
• Kerberos authN, or SSL/GSSAPI (Add Kerberos (SSL/GSSAPI) authN to direct connections #127)