Add ExternalTexture model

Models/ExternalTexture/README.body.md

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+## Screenshot
+
+![screenshot](screenshot/screenshot.png)
+
+## Description
+
+A glTF asset that is stored as binary glTF (GLB file), but still refers to an external PNG file for its texture.
+

Models/ExternalTexture/metadata.json

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+{
+    "version": 2,
+    "legal": [
+        {
+            "license": "CC0",
+            "licenseUrl": "https://creativecommons.org/publicdomain/zero/1.0/legalcode",
+            "artist": "Marco Hutter",
+            "year": "2024",
+            "owner": "Public",
+            "what": "Everything",
+            "text": "CC0 1.0 Universal",
+            "spdx": "CC0-1.0",
+            "icon": "https://licensebuttons.net/p/zero/1.0/88x31.png"
+        }
+    ],
+    "tags": [
+        "core",
+        "testing"
+    ],
+    "screenshot": "screenshot/screenshot.png",
+    "name": "External Texture",
+    "path": "./Models/ExternalTexture",
+    "summary": "A glTF asset that is stored as binary glTF (GLB file), but still refers to an external PNG file for its texture",
+    "createReadme": true
+}