Support setting currency for budget module

[lucascicco]

TL;DR

The budget module does not allow you to set the currency for the specified amount, which may be a problem for some users who want to create a budget based on costs in euros (EUR).

Ref: https://github.com/terraform-google-modules/terraform-google-project-factory/blob/main/modules/budget/main.tf#L71-L75

Terraform Resources

https://registry.terraform.io/providers/hashicorp/google/latest/docs/resources/billing_budget

Detailed design

To avoid any changes that break compatibility, we can add this variable with a default value that returns the USD, as it currently is.

Additional information

No response