Add "Copy file path" button to Diff File header [Rapid Diffs][Diff File]

What does this MR do and why?

For #498890 (closed)

Adds the "Copy file path" button to the diff file header in Rapid Diffs.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Before	Before with FF	After with FF
Screencast_from_2025-01-28_22-01-36	(Feature not implemented)	Screencast_from_2025-01-28_21-58-14

How to set up and validate locally

1. Enable the rapid diffs feature flag in the Rails console
   1. rails c
   2. Feature.enable( :rapid_diffs )
2. Have - or create - an MR with at least 1 changed line
3. View the MR's Changes tab
4. Add ?rapid_diffs=true to the URL, (or: &rapid_diffs=true, if you've already modified the search parameters)
5. Note (& use) the "Copy file path" button
6. (Optional) Verify the copied content is in your clipboard