Create endpoints for discussions on MR rapid diffs

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As noted in #469639 (comment 1985345296), we currently have different implementations for showing discussions on MR and commit pages. We need to create endpoints that can be used for MR and commits diffs in context of rapid diffs.

Based on recent discussion (as of 2024-07-16), we require that the endpoints return the same response format.

This issue is specific for discussion on MR page.