Add duration validator

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

Description:

• Validate Terraform-style durations like 5m, 1h, 30s.

Acceptance criteria:

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