Milo buildbot builder page: Add pagnation with cursors.

milo/api/proto/buildbot.proto

 // Copyright 2016 The LUCI Authors. All rights reserved.
 // Use of this source code is governed under the Apache License, Version 2.0
 // that can be found in the LICENSE file.
 
 syntax = "proto3";
 
 package milo;
 
 import "google/protobuf/timestamp.proto";
 
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 }
 
 // The request for multiple build on a builder.
 message BuildbotBuildsRequest {
   string master = 1;
   string builder = 2;
   // Limit to the number of builds to return (default: 20).
   int32 limit = 3;
   // Include ongoing builds (default: false).
   bool include_current = 4;
+  // Return builds starting from this cursor.
+  string cursor = 5;
 }
 
 // The response message for multiple builds in a builder.
 message BuildbotBuildsJSON {
+  // builds is the list of builds resulting from the builds request.
   repeated BuildbotBuildJSON builds = 1;
+  // The cursor to the next request from, if the exact same query was given.
+  string cursor = 2;
 }