Refactor MergeRequest resource interface used in Rapid Diffs to be consistent

Background

In Rapid Diffs architecture, resources' models (Commit, Compare, MergeRequest) have an almost consistent interface. They respond to:

• #diffs
• #first_diffs_slice
• #diffs_for_streaming
• #diffs_for_streaming_by_changed_paths

For MergeRequest though, we use #latest_diffs when fetching all diffs for a specific merge request. This is because we can't re-use and modify MergeRequest#diffs given its other use cases.

Proposal

Based on !221876 (comment 3075990995) from @slashmanov :

WDYT of creating a delegated VersionedMergeRequest that would have the extended diff method and delegate the rest to merge request? We could use that as our primary resource in the presenter.

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The following discussion from !221876 (merged) should be addressed:

• @slashmanov started a discussion: (+2 comments)

  @patrickbajao WDYT of creating a delegated VersionedMergeRequest that would have the extended diff method and delegate the rest to merge request? We could use that as our primary resource in the presenter.