Create a built-in function to get the GCP project id

[bschaatsbergen]

Similar to the existing AWS built-in functions (e.g. get_aws_account_id(), it would be nice to add some more GCP built-in functions.

To start with, getting the GCP project ID. If I recall correctly FindDefaultCredentials returns Credentials which contains a ProjectID.

Proposed new built-in function: get_gcp_project_id()