Support subject modes

[minionOfZuul]

• Add support for changing mode: READWRITE, READONLY, IMPORT. Docs
• Add support for specifying the id and version when creating a schema subject version. Docs

Several more tests are needed before this work can be called complete.

[minionOfZuul]

I'm fighting with a failure I just can't get to the bottom of, and wanted to ask for help. Test shouldRegisterSchemaForNewResource in SchemaRegistrySubjectControllerTest is failing because the client never makes the request to register the schema subject version.

I have narrowed the cause of the failure to the change here. What's happening is that the Executor is finishing its try-with-resources block and the client is being closed, but this happens before the call to registerSubjectVersion is finished. The change to CreateSchemaSubjectChangeHandler is necessary because the mode needs to be updated before a new version is registered.

Hopefully I'm missing something simple, but I've banged my head on this for several hours now with no luck.