Add signcla verifier for v8.

Currently this verifier will post an informative error if the author has not signed CLA, instead of rejecting the CL.

LOG=N

BUG=457428

Review URL: https://codereview.chromium.org/1185193009

Cr-Commit-Position: refs/heads/master@{#29096}

infra/config/cq.cfg

@@ -46,5 +46,7 @@ verifiers {
       builders { name: "v8_win_rel" }
     }
   }
+
+  sign_cla {}
 }