build: prevent framing of dev app with X-Frame-Options (#24651)

Prevent the dev app site from being place in an iframe.

Author: josephperrott

firebase.json

@@ -14,6 +14,10 @@
           {
             "key": "Cache-Control",
             "value": "no-cache"
+          },
+          {
+            "key": "X-Frame-Options",
+            "value": "DENY"
           }
         ]
       }