Addition of parameter kms_key_self_link in gcp_compute_instance.

##### SUMMARY
When using gcp_compute_instance module, parameter kms_key_self_link is missing.

Currently, `disk_encryption_key` and `source_image_encryption_key` parameters of the **gcp_compute_instance** only offers "raw_key" and "rsa_encrypted_key" options.

**Customer Explaination:**

`disk_encryption_key` and `source_image_encryption_key` parameters of the gcp_compute_instance don't offer any `kms...` option, only directly the key itself (raw or rsa encrypted).

##### ISSUE TYPE
- Feature Idea

##### COMPONENT NAME
gcp_compute_instance

Start with the gcp_compute_instance module and inspect the existing disk_encryption_key and source_image_encryption_key parameter handling. Add support for kms_key_self_link in both parameters, then verify that the module accepts and uses the new option alongside raw_key and rsa_encrypted_key.

Written by the indexing model from the issue text.