Add read-only feature flag for sidekiq memory killer

What does this MR do and why?

It introduces sidekiq_memory_killer_read_only_mode featur flag. It allow us to turn off restarts for sidekiq memory killer. This way, we can safely fine-tune SIDEKIQ_MEMORY_KILLER_GRACE_TIME and SIDEKIQ_MEMORY_KILLER_MAX_RSS, see when memory killer kicks in, make sure that it's not configured to trigger too aggresively, and enable restarting process again.

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

MR acceptance checklist

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Related to #374754 (closed)