Add command to show merged YAML for CI config

Problem to solve

As a developer, I want to be able to view the merged YAML for my GitLab CI configuration inside my VSCode workspace, so that I don't have to leave my IDE and navigate through a web browser to see the results.

Proposal

Utilize the API endpoint to Validate a CI YAML configuration with a namespace, retrieve the merged_yaml attribute from the response, and display the results in a separate editor.

Further details

For complex pipelines, it can be very challenging to know whether the resulting merged YAML is going to look as desired/expected. Having the ability to view this directly in the developer's IDE would be extremely valuable.

Links / references

Might be blocked by gitlab#320778 (closed).

We can use virtual documents to show the merged result: https://code.visualstudio.com/api/extension-guides/virtual-documents.

• We'd define a TextDocumentContentProvider that will serve documents on a custom scheme URI (e.g. gitlab-merged-ci-yaml)
  • this provider would somehow have to store the API response
• We'd show the file by running vscode.workspace.openTextDocument(customUri)