feat: add Configuration settings

Author: averikitsch

gemini-extension.json

@@ -12,5 +12,12 @@
       ]
     }
   },
-  "contextFileName": "DATAPLEX.md"
-}
+  "contextFileName": "DATAPLEX.md",
+  "settings": [
+    {
+      "name": "Project ID",
+      "description": "ID of the Google Cloud project",
+      "envVar": "DATAPLEX_PROJECT"
+    }
+  ]
+}