Introduce GitLab Duo scope for personal access tokens

Problem to Solve

Currently many of the IDE extensions require a PAT with api scope which provides complete read/write access to GitLab. GitLab Duo Code Suggestions only provides data back to the user from the generative AI service. Users don't write anything to GitLab to make this happen.

Proposal

We should introduce a new scope to personal access tokens called ai_features which can be a scope that would allow users in extensions access to receive information from GitLab's AI services (Code Suggestions now, chat and maybe more in the future), but doesn't require full write privileges of the user.