TDF and TDF script changes (application changes pending):

- Added new `pad` element to TDFs
- Refactored `pin` and `signal` elements to accommodate new `pad` element
- Improved validation of signal-to-pad relation in TDF validation script
- Added key attribute to `variant` element
- Removed `package` attribute from `variant` element

build/scripts/Targets/TargetDescriptionFiles/Pad.php

@@ -0,0 +1,14 @@
+<?php
+namespace Targets\TargetDescriptionFiles;
+
+class Pad
+{
+    public ?string $key = null;
+    public ?string $name = null;
+
+    public function __construct(?string $key, ?string $name)
+    {
+        $this->key = $key;
+        $this->name = $name;
+    }
+}