Support for GCS module sources

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Feature description

I realized today that Dependabot for Terraform is able to handle _all_ [Terraform module source](https://www.terraform.io/language/modules/sources) except GCS, so I was wondering if there is any specific reason for it.

We had to find out the hard way after realizing that some repositories haven't been receiving Pull Requests at some point, while others have.

Looking at the [code mentioned in the error message](https://github.com/dependabot/dependabot-core/blob/main/terraform/lib/dependabot/terraform/file_parser.rb#L269) there seem to be cases for all URLs except the ones starting with `gcs::`:

https://github.com/dependabot/dependabot-core/blob/6c40d8d094fb0704cc947485dddb2545b5d1624f/terraform/lib/dependabot/terraform/file_parser.rb#L261-L269

I'd be happy to create a PR to land support for this in Dependabot.
Therefore I would like to know if there are more places this needs to be implemented, or if adding a new case would suffice.
My Ruby is a bit Rust-y (pun intended).

EDIT: found at least one more place that probably needs to be adjusted:

https://github.com/dependabot/dependabot-core/blob/6c40d8d094fb0704cc947485dddb2545b5d1624f/terraform/lib/dependabot/terraform/file_parser.rb#L174-L176

Start in terraform/lib/dependabot/terraform/file_parser.rb at the referenced lines 174-176 and 261-269, and trace how existing Terraform module source URLs are handled. The work is complete when GCS sources beginning with gcs:: are supported in every relevant parser path instead of producing the reported error.

Written by the indexing model from the issue text.