Add ability to validate based off offering

Summary

This change adds a new validation rule that checks whether a configuration value is allowed based on which GitLab platform the user is running — either GitLab.com (SaaS) or a self-managed/dedicated instance.

When a user provides a value in their Terraform configuration, the validator looks at which GitLab environment they're connected to and checks the value against the appropriate list of allowed options. If the value isn't permitted for their platform, they receive a clear error message explaining what went wrong and what values are valid. Values that are empty or not yet known are skipped without error.

The accompanying tests verify all key scenarios: valid and invalid values for both SaaS and self-managed environments, as well as cases where the value is absent or undetermined.

Closes #6757 (closed)