Add ambientCG importer and extract shared web-asset import framework

New Tools/ambientCG Importer window (catalog browser, ZIP download with
selective map extraction, URP/Lit material build). Generic pieces moved
from the PolyHaven tool into Editor/WebAssets (EditorWebRequest,
PbrMapType, PbrMaterialBuilder, ImportUtil, AssetBrowserGrid); PolyHaven
API/window now delegate to the shared code, behavior unchanged.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Assets/Scripts/Editor/AmbientCg.meta

@@ -0,0 +1,8 @@
+fileFormatVersion: 2
+guid: fa6fff02eb207e748b98bea4ebc83bf0
+folderAsset: yes
+DefaultImporter:
+  externalObjects: {}
+  userData: 
+  assetBundleName: 
+  assetBundleVariant: