Support provider-defined functions

[bgpat]

Fixes #213

This PR addresses an issue with the terraform_unused_required_providers rule, which does not currently support provider-defined functions introduced in Terraform 1.8. As a result, valid configurations using these functions were incorrectly flagged as unused providers.

Changes:

• Updated the GetProviderRefs function to correctly recognize provider references in provider-defined functions by walking through expressions and identifying function calls with the provider:: prefix.
• Added logic to extract provider names from these function calls and include them in the set of used providers, preventing false positives in the terraform_unused_required_providers rule.
• Additionally, since GetProviderRefs is also utilized by the terraform_required_providers rule, this update enables the correct detection of provider-defined functions in this rule as well.

[bendrucker]

Very nice! For reference here is Terraform adding the provider functions:

https://github.com/hashicorp/terraform/blob/b676aeb6941b234a6dd58bb2fb59d2cfd3e5abe2/internal/lang/functions.go#L231

And a case where it parses them for improved error diags:

https://github.com/hashicorp/terraform/blob/b676aeb6941b234a6dd58bb2fb59d2cfd3e5abe2/internal/lang/eval.go#L500-L582

Which helps confirm that :: as a function namespace is a Terraform construct. And we can safely parse just by splitting.