Add Experimentation.enabled_for_attribute? method

What does this MR do?

This MR adds the Gitlab::Experimentation.enabled_for_attribute? method for reliably determining whether a certain experiment is active for a given attribute value. It also adds the invitation_reminders key as an experiment.

In an effort to split up the work in !41333 (closed) this MR is kept small.

Related Issue: gitlab-org/growth/team-tasks#191 (closed)

Note: Since this feature will be used behind a feature flag, no changelog was added