[GCP] Allow setting network tag

Hi Team,

One feature our team is interested in is to provide network tags when creating the cluster. The network tag is useful when we configure the firewall rules, so they can be restricted to dask instances only. Currently, these tags are hard coded to http-server and https-server [here](https://github.com/dask/dask-cloudprovider/blob/41e6624def02526cc44043a847e9f9ec6bc942bd/dask_cloudprovider/gcp/instances.py#L115)

Start in dask_cloudprovider/gcp/instances.py at the linked line where network tags are hard coded to http-server and https-server. Trace how the GCP cluster is created and identify where a configurable tag value belongs. Done means callers can provide network tags during cluster creation instead of relying only on those hard-coded tags.

Written by the indexing model from the issue text.