Implement ChangeSubjectCompatibilityLevel, GetGlobalCompatibilityLevel and GetCompatibilityLevel

[doxsch]

I have implemented the following two endpoints to be able to read the compatibility levels.

• https://docs.confluent.io/platform/current/schema-registry/develop/api.html#get--config
• https://docs.confluent.io/platform/current/schema-registry/develop/api.html#get--config-(string-%20subject)
• https://docs.confluent.io/platform/current/schema-registry/develop/api.html#put--config-(string-%20subject)

[AtakanColak]

First of all thank you for your PR. I'm just hesitant to add this functionality unless it is ever needed or requested.

[doxsch]

First of all thank you for your PR. I'm just hesitant to add this functionality unless it is ever needed or requested.

I understand your concerns, of course.

We need this functionality in a project. We will fill the schema registry in a Gitops approach. Therefore I reimplemented the more administrative endpoints, which it didn't need before. As far as I could find, this project is the only go client lib for the schema registry of confluent, so I implemented the parts relevant for us.

[AtakanColak]

All right. Approving this one, but you'll need to modify #54 so it doesn't break compatibility.