Merge requests API: fix cache for "with_labels_details" argument

What does this MR do and why?

Contributes to #386784 (closed)

Problem

Merge requests API endpoint uses cache, but the cache refresh doesn't trigger when the user provides with_labels_details argument.

Solution

Add with_labels_details argument value into the cache key. It will create a new version of the cache when the user passes with_labels_details.

MR acceptance checklist

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Before	After

How to set up and validate locally

1. Find or create a project with merge requests
2. Add labels to a merge request
3. Visit http://gdk.test:3000/api/v4/projects/<project_id>/merge_requests
4. You should see labels field to contain a list of strings.
5. Visit http://gdk.test:3000/api/v4/projects/<project_id>/merge_requests?with_labels_details=true
6. You should see labels field to contain with a list of hashes.