fix: add IAM endpoint for us-isob-* (#3056)

Author: trivikr

.changes/next-release/bugfix-iam-b72f6077.json

@@ -0,0 +1,5 @@
+{
+  "type": "bugfix",
+  "category": "iam",
+  "description": "add IAM endpoint for us-isob-*"
+}

lib/region_config_data.json

@@ -3,6 +3,9 @@
     "*/*": {
       "endpoint": "{service}.{region}.amazonaws.com"
     },
+    "us-isob-*/iam": {
+      "endpoint": "iam.{region}.sc2s.sgov.gov"
+    },
     "cn-*/*": {
       "endpoint": "{service}.{region}.amazonaws.com.cn"
     },
@@ -60,4 +63,4 @@
       "signatureVersion": "s3"
     }
   }
-}
+}