[Feature]: Support GCP ALB for gateways

[jvstme]

Problem

There is no support for managed Gateway Load Balancers on GCP. Users have to deal with the complexity of creating the load balancer outside of dstack and keeping all gateway replicas registered

Solution

Support GCP ALB (regional). See the existing AWS ALB support for reference.

Workaround

No response

Would you like to help us implement this feature by sending a PR?

Yes