No way to link compute_resource_policy to an actual compute item

##### SUMMARY

We have gcp_compute_resource_policy which allows us to create a snapshot policy, however there's no way to actually bind that policy to a virtual disk.

This probably means adding a parameter to gcp_compute_disk that accepts the policy name.

##### ISSUE TYPE
- Feature Idea

##### COMPONENT NAME

gcp_compute_resource_policy
gcp_compute_disk

Start with the gcp_compute_disk and gcp_compute_resource_policy module entry points, reading how each resource exposes its arguments and relationships. Trace how a snapshot policy name should be passed to a virtual disk, then verify that a disk can be created with the policy bound and that the resource documentation reflects the new parameter.

Written by the indexing model from the issue text.