Add the idea of a regular throttle

What does this MR do and why?

In my MR to add another throttle (!70310 (merged)), I'm hitting a Rubocop Metrics/AbcSize error in Gitlab::RackAttack.configure_throttles , which is the proximate cause of this refactoring.

Currently we have three throttles that are exactly copy-pastes of each other. I'm about to add a fourth, and the web throttle can become a fifth once a couple of columns in the application_settings table are renamed.

To avoid further copy-pasting in what is becoming a popular area of the codebase, I add a "regular throttle" abstraction that lets us just add the throttle fragment / name to an array and get this functionality for free.

Irregular throttles can still be added, but if we can make a throttle regular, we should, to aid us in future refactorings.

MR acceptance checklist

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Related to #337829 (closed)