feat: Add Ruby message schema files

Context on this change

• These files will be copied into the vendor/gems directory of the gitlab monolith so that they can be used to instantiate a Ruby client to interact with the Duo Workflow Service.
• We may choose to publish this in rubygems.org later. For now, we are generating these files using the proto definitions in this project and will just manually copy the files over.
• These files were generated using the following command while in the root directory of the duo_workflow_servic project:

grpc_tools_ruby_protoc -I contract --ruby_out=clients/ruby/lib/proto --grpc_out=clients/ruby/lib/proto contract/contract.proto

Note that I did have to make one small change to these generated files: I made to these generated files is that I addedrequire_relative to L5 of contract_services_pb.rb. Auto-generated version just had require. Before this change, I received an error. This appears to be a known issue. See https://github.com/grpc/grpc/issues/29027

How to test this change

See my branch on the monolith where I did the following:

1. Run the Duo Workflow Service server: poetry run python -m duo_workflow_service.server
2. Copy the files added in this MR into the vendor/gems directory of gitlab
3. Add the vendored gem to the Gemfile
4. Invoke the client I created in ee/lib/ai/duo_workflow_service/client.rb in a Rails console: Ai::DuoWorkflowService::Client.new(duo_workflow_service_url: "localhost:50052").generate_token