Add user actor for Zoekt API

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Problem to solve

From internal thread: https://gitlab.slack.com/archives/C3TMLK465/p1726852223664909 zendesk ticket: https://gitlab.zendesk.com/agent/tickets/569245

zoekt_search_api feature flag is gated by root namespace_id

That means the whole namespace uses zoekt for API code search results when it's enabled. This does not allow users to test the new API and adapt to it before fully switching over.

Proposal

Related: gitlab-com/gl-infra/production#17522 (closed)

Add a second feature flag gated by user: zoekt_search_api_by_user. We can turn the flag on for a specific user OR the whole root namespace