Add support to windows-startup-script-ps1 on Packer GCP (Terraform already supports this feature)

_This issue was originally opened by @lmayorga1980 in https://github.com/hashicorp/packer/issues/10342 and has been migrated to this repository. The original issue description is below._

---

#### Description

According to https://www.packer.io/docs/builders/googlecompute.html#windows, is only possible to use the `windows-startup-script-cmd` when using GCP as the builder. Would it be possible to use an existing powershell script?

#### Use Case(s)

Powershell script uses a combination of `cmdlets` and `native windows` executables.

#### Potential configuration

```
metadata": { "windows-startup-script-ps1": "scripts/user_data.ps1" },
```