Add user to Snowplow event

What does this MR do and why?

Adds user_id to GitLab standard context for Snowplow tracking. This change is behind feature flag until this changes are approved. Feature flag is actor based (can be enabled for chosen users). Closes #338150 (closed)

How to set up and validate locally

1. Enable feature flag
2. Track an event

Feature.enable(:add_actor_based_user_to_snowplow_tracking, User.first)
Gitlab::Tracking.event('a', 'b', user: User.first)

3. user_id will be added to standard context payload

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.