[Feature] GCP backend: allow specifying the subnetwork (currently always picks the first usable subnet in the VPC) #4254

Problem

The GCP backend lets you pin the VPC via vpc_name, but not the subnet. Subnet selection is:

# src/dstack/_internal/core/backends/gcp/resources.py
def get_vpc_subnet_or_error(...):
    vpc_subnets = get_vpc_subnets(vpc_name, region, usable_subnets)
    if vpc_subnets:
        return vpc_subnets[0]   # <- first usable subnet, no way to influence

i.e. the first subnet returned by ListUsableSubnetworks for that VPC+region wins.

In a VPC with multiple subnets in the same region, subnets often serve different purposes (different sizes, routing, peering/VPN reachability, reserved ranges), and you may want dstack to provision instances and gateways in one subnet rather than another. Today there is no way to influence the choice — which subnet dstack lands in is effectively arbitrary. This is especially limiting for private gateways and fleets with public_ip: false, where clients can only reach the VMs if they land in a subnet the client network routes to.

Proposal

Add an optional subnetwork field to the GCP backend config (alongside vpc_name), e.g.:

- type: gcp
  project_id: my-project
  vpc_name: my-vpc
  subnetwork: my-subnet   # optional; per-region map also fine
  creds:
    type: default

When set, get_vpc_subnet_or_error would select that subnet (erroring if it doesn't exist in the region) instead of vpc_subnets[0]. Happy to contribute a PR if the approach sounds right.

dstack version: 0.21.3 (server, GCP backend; behavior unchanged on master)