[RFE] Support Shared VPCs

[agrare]

Currently the google provider refresh does not collect cloud networks / subnets from Shared VPCs (https://cloud.google.com/vpc/docs/shared-vpc) because these come from other projects.

We have to first query for the shared VPC host project (if it exists) with https://cloud.google.com/compute/docs/reference/rest/v1/projects/getXpnHost

Then we can list the networks / subnets on that project that are shared.

NOTE fog-google doesn't support these APIs so we will have to use https://github.com/googleapis/google-api-ruby-client