Add comparing diff versions and specific commit support for MR diffs streaming endpoint

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Background

We decided to keep the initial implementation of streaming endpoint for MR diffs simple. As discussed in #450990 (comment 1901990588), we are splitting adding support for existing features that diffs_batch.json supports.

This issue is for supporting comparing different diff versions and specific commit. Currently, the streaming endpoint is only getting the diff of latest base diff.

Proposal

Update MergeRequests::DiffsStreamController to support comparing different diff versions and specific commit.