gcp_compute_region_disk ignores source_snapshot parameter

##### SUMMARY
When using the `gcp_compute_region_disk` module with the `source_snapshot` parameter, the parameter is ignored and an empty disk is created.
I believe this is due to the absence of `source_snapshot` from this modules `resource_to_request` function:
https://github.com/ansible-collections/google.cloud/blob/eec40d5ca1f684e22134f2613482e36ed4ecd17c/plugins/modules/gcp_compute_region_disk.py#L508

See the `gcp_compute_disk` module, which does work:
https://github.com/ansible-collections/google.cloud/blob/eec40d5ca1f684e22134f2613482e36ed4ecd17c/plugins/modules/gcp_compute_disk.py#L665

##### ISSUE TYPE
- Bug Report

##### COMPONENT NAME
gcp_compute_region_disk

##### ANSIBLE VERSION
```paste below
2.14.18
```

##### COLLECTION VERSION
```paste below
1.1.3
```

##### CONFIGURATION
None changed

##### OS / ENVIRONMENT
Docker

##### STEPS TO REPRODUCE
Create a disk using the following snippet.
Inspect the created disk and see that it has no source snapshot.

```yaml
- name: Create disk
gcp_compute_region_disk:
project: my-project
name: my-disk
source_snapshot:
selfLink: "{{ data_disk_snapshot.resources[0].selfLink }}"
region: europe-north1
replica_zones:
- europe-north1-a
- europe-north1-b
```

##### EXPECTED RESULTS
A disk should be created from the specified source snapshot.

##### ACTUAL RESULTS
An empty disk was created, the `source_snapshot` parameter is not present in the GCP resource spec.

Start in plugins/modules/gcp_compute_region_disk.py at the resource_to_request function referenced in the issue, then compare it with the corresponding function in gcp_compute_disk.py. Reproduce the YAML example against GCP and inspect the created regional disk; done means source_snapshot is included in the resource request and the disk is created from that snapshot.

Written by the indexing model from the issue text.