Add StackProf to the Gemfile, along with a utility to get a profile for a spec

What does this MR do?

Adds stackprof to the development / test environment, along with a script to do an rspec run under stackprof

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

Why was this MR needed?

The test suite needs speeding up significantly. This is one tool for investigating it.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Tests
  • 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?

Related to #23034 (closed)