Simplified the build of the spec

[jieyu]

The original build is unnecessarily complex. This patch simplifies the
build process by just generating the protobuf file.

[jdef]

I really liked having the generated artifacts, can we keep these? (requires the earlier gem install ...)

[akutz]

Hi @jieyu,

I don't see the value in this patch. The original build works fine and the external csi.mk file makes it easy for others to use that locally.

[jieyu]

Did an offline chat with @akutz and @saad-ali. We will proceed with the following plan:

1. Make protobuf the source of truth
2. Reference the protobuf in the spec.md
3. Remove libraries repo in the csi org
4. Introduce a lib/ folder in the spec repo
5. Putting language specific libraries (used for spec validation) in lib/ folder. First step will be a minimal client based on generated grpc code and a mock server. It'll try to validate the workflow specified in the spec doc.

cc @saad-ali