Persist and use truncated note diffs instead requesting Gitaly in a N+1 manner on MR page

What does this MR do?

Creates a new table to persist a truncated diff (i.e. get rid of the diff below every discussion comment and persist it). This relation is a simple 1-1 with notes.

This is a similar approach of GitHub's diff_hunk.

Why was this MR needed?

We request Gitaly in a N+1 manner to build the discussion diffs. Once the diffs are from different revisions, it's hard to make a single request to the service in order to build the whole response.

With this change we solve this problem and simplify a lot fetching this piece of data.

Statistics

On MR with ~15 discussions on diffs:

Before:

• 125ms submitting 51 Gitaly RPC calls
• 24ms submitting 52 PG queries

After:

• 52ms submitting 15 Gitaly RPC calls
• 25ms submitting 53 PG queries

This means we're now submitting 15 requests where we were submiting 51 before. We can also see a 140% improvement on RPC timing.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a Backend maintainer
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #45190 (closed)