Verify method in GitlabShellWorker before using `public_send`

What does this MR do and why?

There are only a few valid methods that this could be calling on Gitlab::Shell, so we explicitly list them and check in that list before calling public_send.

This has a feature flag, disabled by default, to make failing that check raise ArgumentError.

A Sidekiq job class that takes a method name as an argument and calls that method (with public_send on an object) is a security risk. It's not typically directly exploitable, but it can be used to escalate from Redis injection to arbitrary code execution (see #371098 (closed) for an example).

We should remove public_send from Sidekiq workers where possible, and where not possible, validate the method name we're passing carefully. I found these workers that disable the Security/PublicSend cop:

Split from !103053 (closed). For #371470 (closed).

How to set up and validate locally

Mostly just specs.

MR acceptance checklist

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