Introduce execute_background_migrations feature flag

What does this MR do and why?

This MR add new execute_background_migrations feature flag of type ops, to provide a way to pause classic (sidekiq based) background migrations. When the feature flag is disabled, jobs are not execute but re-scheduled in the future.

See #363567 (comment 964870636) for more context.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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

Related to #363567 (closed)