Resolve "Improve efficiency of serialized diff caching for merge requests"

What does this MR do and why?

This enables HTTP caching using ETag.

The overall issue is going to involve removing the redis caching that exists, but firstly we will introduce ETag caching, see if any performance issues, then another MR will be made to put the redis caching behind a feature flag.

This is behind the etag_merge_request_diff_batches feature flag.

How to set up and validate locally

1. Enable etag_merge_request_diff_batches feature flag.
2. Create a MR.
3. Load the MR page with console open. Check the Network tab. Ensure that "Disable cache" is unchecked.
4. Reload the page and should see requests for diff_batch.json return 304.

Related to #367145 (closed)