Milo buildbot builder page: Add pagnation with cursors.

milo/api/proto/buildbot.proto

Index: milo/api/proto/buildbot.proto
diff --git a/milo/api/proto/buildbot.proto b/milo/api/proto/buildbot.proto
index 07c276f1f4376c3252c7adece55ab6acc33215c4..d1210812d0e5634323a083b16274717822d9cbc9 100644
--- a/milo/api/proto/buildbot.proto
+++ b/milo/api/proto/buildbot.proto
@@ -53,9 +53,14 @@ message BuildbotBuildsRequest {
   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;
 }