Add new Sidekiq MemoryKiller Watcher with NullHandler

What does this MR do and why?

Based on proposal, it Implements new Gitlab::Memory::Sidekiq::MemoryKiller that will replace current Gitlab::SidekiqDaemon::MemoryKiller

• ImplementsGitlab::Memory::Sidekiq::MemoryKiller::Watcher which is responsible for monitoring RSS of the Sidekiq process
• Implements Gitlab::Memory::Sidekiq::MemoryKiller::NullHandler which will be the default handler for handling high RSS. It will not restart/terminate the process, and it would allow us to safely fine-tune the soft limits.

Sidekiq::MemoryKiller::Watcher will not use deprecated Daemon class, it run within BackgroundTask

Gitlab::BackgroundTask.new(Gitlab::Memory::Sidekiq::MemoryKiller::Watcher.new).start

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

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 #370239 (closed)