Add equals validator

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

Description:

• Validate that two values are equal.

Acceptance criteria:

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