Add `feature_flag` domain expertise

Why is this change being made?

I've been actively involved with Feature flags in the development of GitLab for a few years. For example,

• I'm mentioned in the blueprint as the Domain Expert.
• I've contributed many MRs to improve the process:
  • gitlab-org/gitlab!71254 (merged)
  • gitlab-org/gitlab!64696 (merged)
  • gitlab-org/gitlab!61586 (merged)

Given that feature flags are important tools to ensure our deployment safety, I think it makes sense to introduce the domain in data/domain_expertise.yml. This would be helpful for new developers to reach out the domain experts when they need an assistance of this process.

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