Get code suggestions token from CustomersDot

Follow-up to #415550 (closed).

Once #416468 (closed) is done, gitlab-rails will be able to obtain a JWT (instance token) from CustomersDot and make requests to the AI gateway (model gateway) to obtain code suggestions for self-managed users.

To enable this, we need to:

• Obtain and refresh this token regularly, most likely via SyncSeatLinkWorker
• Store this token so it can be used to make model gateway requests from the AI abstraction layer

Other considerations:

• The token must be replaced with each sync
• We need to be careful not to break the existing flow, unless we deem this OK considering it's in beta