Adds generated go client for oci-catalog that we'll use from syncer.

[absoludity]

Description of the change

Follows on from #6595, updates the proto service files according to buf's lint, and generates the go client for use in the asset-syncer.

Note: I added a separate buf.yaml and buf.gen.yaml for the oci-catalog service as buf doesn't let you refer to a proto outside of the buf.yaml root, so the other option would have been to move our existing buf.yaml and buf.generate into the Kubeapps root directory. I don't mind either way, but it made more sense to me for the oci-catalog service to have its own so that it is independent (it may be useful outside of Kubeapps, theoretically).

Benefits

The next PR can use the client without being cluttered by the extra diff.

Applicable issues

• ref: Implementation of OCI-Catalog gRPC micro-service #6263

[antgamdia]

+1 to the independent buf.yaml files. However, I would add this to the release docs (under the Protobuf dependencies and autogenerated code section), mainly just not to forget about this component when upgrading our deps.

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[absoludity]

+1 to the independent buf.yaml files. However, I would add this to the release docs (under the Protobuf dependencies and autogenerated code section), mainly just not to forget about this component when upgrading our deps.

Thanks. I added two small makefile targets to handle both projects for us and updated the release docs to use that.