Add a parameter --random to set explicitly when enabling a feature flag with a percentage of time

This adds a new parameter --random that should be set when enabling a feature flag with a percentage of time (unless it's 0 or 100), so that the user is more aware of the implications.

Also I had to fix the shared examples "invalid feature flag update" in rspec which didn't work as intended for the case when enabling in staging.

See #107 (closed)

Corrective action for https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/14974