Add REST endpoint for compatibility check

[granders]

Spec calls for REST endpoint to allow compatibility check, but this is not yet implemented.

[nehanarkhede]

@granders Don't remember, is this already checked in?

[granders]

@nehanarkhede This was checked in and merged (https://github.com/confluentinc/schema-registry/tree/compatibility-check)

However, I did just notice that the global-schema-id branch does not include these changes.

[nehanarkhede]

@granders Ah, my bad. I guess that's solvable via a merge. So this issue can be closed?

[granders]

Hmm, how about I try merging in master before closing?

[nehanarkhede]

@granders Yup, no problem. Whatever is easier.

[granders]

Complete, but rendered obsolete by #48