Add spec back and make standard

Author: hamishwillee

http/headers/Idempotency-Key.json

@@ -3,6 +3,7 @@
     "headers": {
       "Idempotency-Key": {
         "__compat": {
+          "spec_url": "https://datatracker.ietf.org/doc/html/draft-ietf-httpapi-idempotency-key-header/",
           "support": {
             "chrome": {
               "version_added": false
@@ -35,7 +36,7 @@
           },
           "status": {
             "experimental": true,
-            "standard_track": false,
+            "standard_track": true,
             "deprecated": false
           }
         }