Add not_in_list validator

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

Description:

• Validate that a string does not match any value from a list.

Acceptance criteria:

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