Call to undefined method in `SchemaRegistryClient.post_new_schema()`

### What happened?

Discovered when working on static type checking coverage.

On this line there's a call to `references.json()` where `references: Reference`, however `Reference` has no `.json()` method.

https://github.com/aiven/karapace/blob/42b14c71543b878c42c11025ab266122d61550d3/karapace/serialization.py#L92

I tried adding `raise NotImplementedError` on that line, and confirmed that there is no test coverage there. Presumably, a case should be added to `tests/integration/test_client_protobuf.py`.

### What did you expect to happen?

- Test suite should be failing.
- Correct method should be called.