Add MutualTLSCredentials as a placeholder Credentials type

[rohitsanj]

Followup from #201

Implement a new Credentials type called MutualTLSCredentials containing just a single field derived that is always set to true.

The SSL configuration for using mTLS will still need to be passed in the kafka_cluster.ssl (or schema_registry.ssl) object.

The direct connection form in vscode should expose the same fields as what TLSConfig provides if a user selects mTLS as auth mechanism, however, the inputs will be passed in the ConnectionSpec's kafka_cluster.ssl (or schema_registry.ssl) field. The credentials field will perhaps need to be set to just { "derived": true}, this serves as the creds type discriminator (to determine what creds type we're dealing with dynamically based on the fields present) in the Quarkus deserialization logic.