Add country_code validator

Implement a new provider::validatefx::country_code validator.

Description:

• Validate ISO 3166-1 alpha-2 country codes.

Acceptance criteria:

1. Subtask: Implement country_code validator in internal/validators
2. Subtask: Add unit tests for country_code validator
3. Subtask: Add country_code function to internal/functions package
4. Subtask: Add unit tests for country_code function
5. Subtask: Register country_code in registry.go
6. Subtask: Add documentation in examples/functions/country_code/function.tf
7. Subtask: Update docs table using make docs command
8. Subtask: Add integration test in integration/main.tf