Add an optional performance bar to view performance metrics for the current page

EE MR at https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2104:

What does this MR do?

It adds a perf bar (using https://github.com/peek/peek) that can be enabled with the p b shortcut.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots

The bar

SQL queries & duration (sorted by duration DESC) shown in a modal

RBlineprof modal with syntax highlighting with Rouge

Shortcut documented in the shortcuts modal

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #29010 (closed)