Use default devfile with golden image to create workspace if no devfile is present

Issue - Backend: Use default devfile with golden image ... (#474986 - closed)

What does this MR do and why?

As per the plan in Provide default devfile for projects if not pre... (&14644 - closed) , use default devfile with golden image to create workspace if no .devfile.yaml is present in the root of the repository.

References

Epic here Provide default devfile for projects if not pre... (&14644 - closed)

MR acceptance checklist

• DB migration to make devfile_path nullable
• Update devfile_fetcher.rb to take in default devfile is needed
• Rename devfile_ref to project_ref in the DB and the GraphQL and the codebase.
• Add a new settings in the settings module for default_devfile.

Screenshots or screen recordings

Below test scenarios are all with code changes

Scenario	Screenshot	DB record
All existing params
project_ref is used and ignore devfie_ref
project_ref is used with no devfile_ref
no project_ref or devfile_ref
devfile_path is null (highlighted image is different from other screenshot image, and is using default_devfile from settings)
devfile_path is empty string and return error as expected

How to set up and validate locally

1. Setup workspaces locally following this guide
2. Navigate to http://gdk.test:3000/-/graphql-explorer
3. Also find your local correct project id and agent id for the use in the graphql query
4. Execute queries in above screenshot, one example is provided below.
5. Verify results as expected, and also try other different params.

mutation workspaceCreate {
  workspaceCreate(
    input: {
      projectId: "gid://gitlab/Project/3",
      clusterAgentId: "gid://gitlab/Clusters::Agent/10",
      desiredState: "Running",
      devfileRef: "main-invalid",
      projectRef: "main",
      devfilePath: null,
      maxHoursBeforeTermination: 24,
      variables: []}
  ) {
    workspace {
      id
      projectRef
      devfileRef
      devfilePath
      devfile
    }
    errors
  }
}