Add memory_mb validator

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

Description:

• Validate memory values in MB are within allowed increments.

Acceptance criteria:

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