feat(nimbus): Send alerts to the user if enrollment is let 1000

[yashikakhurana]

Because

• Because if the experiment is misconfigured we might not see enrollments, so now we can add a check if enrollment numbers is less than 1000 after 3 days, then we can notify the users about it.

This commit

• Send the slack notification if the enrollment is let 1000

Fixes #15522