Add past_datetime validator

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

Description:

• Validate that a datetime is in the past.

Acceptance criteria:

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