GeezerVison

`dstack` now creates and manages an Application
Load Balancer for GCP gateways when
`load_balancer` is set to `{ type: alb }`.

```yaml
type: gateway
name: gcp-gateway
backend: gcp
region: europe-west9
domain: example.com
load_balancer:
  type: alb
replicas: 2
certificate: null
public_ip: false
```

This allows you to have more than one GCP gateway
replica, with load balancing handled automatically
by `dstack`.

```shell
$ dstack gateway list
 NAME          BACKEND             HOSTNAME     DOMAIN       DEFAULT  STATUS
 gcp-gateway                       10.200.0.53  example.com  ✓        running
    replica=0  gcp (europe-west9)  10.200.0.54                        running
    replica=1  gcp (europe-west9)  10.200.0.55                        running
```

GCP ALB is supported for both `public_ip: false`
and `public_ip: true` gateways. Currently, it is
only supported with `certificate: null` (no
HTTPS).