Automatic merge from submit-queue (batch tested with PRs 45573, 46354, 46376, 46162, 46366)

GCE - Retrieve subnetwork name/url from gce.conf 

**What this PR does / why we need it**:
Features like ILB require specifying the subnetwork if the network is type manual.

**Notes:**
The network URL can be [constructed](https://github.com/nicksardo/kubernetes/blob/68e7e18698d60050924c0f0e8f794a21820f2a1a/pkg/cloudprovider/providers/gce/gce.go#L211-L217) by fetching instance metadata; however, the subnetwork is not provided through this feature. Users must specify the subnetwork name/url through the gce.conf.

Although multiple subnets can exist in the same region for a network, the cloud provider will only use one subnet url for creating LBs. 


**Release note**:
```release-note
NONE
```