Implement block pipl users

What does this MR do and why?

This MR introduces the blocking of PIPL-Compliant users after the notice period is over.

The detailed changes are the following:

• Introduce a cron worker that fetches the eligible users and blocks them using the internal admin user
• Introduce a service that validates and initiates the user blocking
• Add a user note in order to ease the work of our support engineers
• A custom attribute is added since we use the already-existing ::Users::BlockService

Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/455708

The user deletion will be implemented in a follow-up MR

References

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