Add API::CodeSuggestions to OpenAPI v2 Documentation

Summary

We want to add our Code Suggestions API Endpoints to the OpenAPI Docs YML. This will result in our endpoints being available in that interactive documentation for use. It is "optional" in practice (see the initial section Documenting REST API Resources ). Nonetheless, long-term having our endpoints here could be beneficial to consumers of the product.

Solution

1. Mount the Code Suggestions namespace in lib/api.rb
2. Generate an updated OpenAPI YML using the gitlab:openapi:generate rake task.

Note: Because we do not generate this YML file with each iteration of introducing a new endpoint, for example, the resulting MR may include more changes to the file.