This commit implements provisioning GCP A4
clusters with high-performance RoCE networking.

```shell
> dstack fleet
 FLEET  INSTANCE  BACKEND         RESOURCES                                          PRICE    STATUS  CREATED
 gpu    0         gcp (us-west2)  cpu=224 mem=3968GB disk=100GB B200:180GB:8 (spot)  $51.552  idle    21 mins ago
        1         gcp (us-west2)  cpu=224 mem=3968GB disk=100GB B200:180GB:8 (spot)  $51.552  idle    17 mins ago
```

To enable high-performance networking, users need
to create the
[appropriate networks](https://cloud.google.com/ai-hypercomputer/docs/create/create-vm#setup-network)
and configure them in the backend settings.

```yaml
projects:
- name: main
  backends:
  - type: gcp
    project_id: my-project
    creds:
      type: default
    vpc_name: my-vpc-0  # regular, 1 subnet
    extra_vpcs:
    - my-vpc-1  # regular, 1 subnet
    roce_vpcs:
    - my-vpc-mrdma  # RoCE profile, 8 subnets
```

Then apply a fleet configuration.

```yaml
type: fleet
nodes: 2
placement: cluster
availability_zones: [us-west2-c]
backends: [gcp]
resources:
  gpu: 8:b200
```

Each instance in the cluster will then have 10
network interfaces:
- 1 regular interface in the main VPC (`default`
  or the one configured in `vpc_name`).
- 1 regular interface in a VPC configured in
  `extra_vpcs`.
- 8 RDMA interfaces in the VPC configured in
  `roce_vpcs`.

Additionally, this commit optimizes the fetching
and caching of subnets, so that they are fetched
from the API only once, and not separately for
each item in `extra_vpcs`. For some instance
types, this reduces the number of API requests
from 9 to 1, which cuts about 16 seconds from each
offer provisioning attempt.