`gcp_compute_snapshot` doesn't support sharing snapshots across projects

##### SUMMARY
The gcloud API supports creating a snapshot in a different project from the `source_disk`, but the `gcp_compute_snapshot` module does not.
source: https://cloud.google.com/compute/docs/disks/create-snapshots#sharing_snapshots

##### ISSUE TYPE
- Feature Idea

##### COMPONENT NAME
gcp_compute_snapshot

##### ADDITIONAL INFORMATION
This is useful for migrating VMs between projects. It requires replacing the API call [disks.createSnapshot](https://cloud.google.com/compute/docs/reference/rest/v1/disks/createSnapshot) with [snapshots.insert](https://cloud.google.com/compute/docs/reference/rest/v1/snapshots/insert)

Start with the gcp_compute_snapshot module and locate its use of the disks.createSnapshot API. Read the Google Cloud snapshots.insert documentation and the linked sharing-snapshots guidance, then verify that the module can create a snapshot in a project different from the source disk's project.

Written by the indexing model from the issue text.