Add FF to avoid directly accessing diffs via gitaly

What does this MR do?

The Merge Request Changes API exposes the full diff by going directly to Gitaly, avoiding the existing DB-backed diffs (#225322 (comment 378507185)) This MR causes us to prefer the DB-backed diffs, but as those diffs are potentially limited to certain max number of files, bytes, etc, we use a feature flag to allow impacted projects who are willing to accept potentially non-performant requests to use the existing legacy approach that they had previously relied upon.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

Related to #225322 (closed)