Support newer Schema Registry features

[minionOfZuul]

Is your feature request related to a problem? Please describe.
I need to keep several schema registry instances in sync with a master registry. Confluent schema linking is not an option. In order to do this with jikkou, the schema registry provider needs to have support for a few newer features.

Describe the solution you'd like
Here are the missing features:

• support for changing modes (IMPORT, READWRITE, READONLY).
• support for registering a schema with a specified ID (The only docs for this I could find are here)
• probably optional: first class support for schema registry contexts (docs here. Note, this isn't a feature of Confluent's proprietary exporter implementation, but is baked into schema registry itself)

Describe alternatives you've considered
We've tried to use Confluent schema linking for this, but it's too opaque (can't see the logs) and there are too many ways to misconfigure it.

Additional context
I'm willing to take a shot at implementing PRs for these features, but I'd like to discuss design first, especially around registering with a specified schema ID. I've noticed that there is an annotation that jikkou adds to the SchemaRegistrySubject resource that contains the ID, but it doesn't appear that it's used as an input as well. Is that an idea worth exploring?

[fhussonnois]

Hi @minionOfZuul, your use case is very interresting, and of course any contribution will be welcomed. You are right, for the moment the annotation schemaregistry.jikkou.io/schema-id is only used when collecting schemas. For your need, it seems logical to use it instead of modifyng the resource schema.

[minionOfZuul]

Here's my first attempt at a PR for this feature: #397. It's currently a draft - please see my comment there.