support Retry-After header

https://github.com/acmesh-official/acme.sh/issues/2939

acme.sh

@@ -4517,6 +4517,12 @@ $_authorizations_map"
         _info "Pending"
       elif [ "$status" = "processing" ]; then
         _info "Processing"
+        _retryafter=$(echo "$responseHeaders" | grep -i "^Retry-After *:" | cut -d : -f 2 | tr -d ' ')
+        _debug "_retryafter" "$_retryafter"
+        if [ "$_retryafter" ]; then
+          _info "Retry after: $_retryafter"
+          _sleep $_retryafter
+        fi
       else
         _err "$d:Verify error:$response"
         _clearupwebbroot "$_currentRoot" "$removelevel" "$token"