Introduce a new flag for "Selectively disable by actor" in addition to the Chat Agent V2 feature flag

Problem

@tlinz Yes, please ask them to help what @brytannia mentioned above.

We've made a estimated timeline to complete the V2 switching.

It's most likely we can't make %17.4 if we play safe. One way to shorten this timeline is to rollout the V2 switching to GitLab team members immediately. This could bother Sales and SAs so we need to introduce Selectively disable by actor to disable the flag for an urgent need. Simply put, we sacrifice ourselves to get the feedback as soon as possible. It's likely that there are undiscovered bugs that could be a blocker against global rollout.

Proposal

Introduce https://docs.gitlab.com/ee/development/feature_flags/controls.html#selectively-disable-by-actor.