Add TimeoutHelpers

What does this MR do and why?

This is Step 1 of !101258 (closed)

This MR splits some migration helpers in dedicated module and introduces the following:

• Gitlab::Database::Migrations::TimeoutHelpers

We are only moving files around, all features remain untouched.

Why are we doing this?

migration_helpers.rb has 1734 lines and a lot of helpful methods used by developers to write migrations.

This MR proposes to split the responsibility between different migration helpers which are still included within our main migration helper.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.