Send "dedicated" as realm value if applicable

Problem

With https://gitlab.com/gitlab-org/iglu/-/merge_requests/123 we have dedicated as a potential third value for realm, but currently, dedicated instances are still sending self-managed as the value for realm when sending events from the gateway.

Desired Outcome

A dedicated instance sets realm correctly to dedicated when sending events.

Potential Solution

• Understand how we can identify Dedicated customers in CDot
• Find and patch code in any participating component (gitlab, CDot, AIGW, CC library) that acts on gitlab_realm == self-managed, which would not be correct anymore because it would now exclude dedicated customers (not certain we have such code but I think we do)
• Update CDot to write dedicated to the gitlab_realm claim when minting tokens for Dedicated instances.
• Update gitlab-rails to send the correct x-gitlab-realm HTTP header (must match the token claim), so we may have to have this logic here too

How to verify

We have events with dedicated as value for gitlab_realm/realm

Further actions needed