Add missing Schema Registry ACLs

[solita-juusoma]

• Please check if the PR fulfills these requirements

• The commit messages are descriptive
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• An issue has been created for the pull requests. Some issues might require previous discussion.

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
  Bug fix: Add missing Schema Registry ACLs (Schema Registry ACLs missing #296)

Adds field consumer_offsets_topic under platform schema_registry instances.

platform:
  schema_registry:
    instances:
      - principal: "User:schema_registry"
         topic: "foo"
         consumer_offsets_topic: "foo_offsets"  # new field
         group: "bar"

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)

Change is now done so that it won't break anything but introduces new fields and additional ACLs that are required for schema-registry principal.

• Other information:

topic field is not that descriptive. Should be something like schemas_topic but changing it would make it breaking change. I am not sure is somebody using the functionality currently because I don't see how it would work (without GROUP and required ACLs) ? Therefore could be possible to change the name

[purbon]

LGTM, thanks for your contributions @solita-juusoma