Milo buildbot builder page: Add pagnation with cursors.

milo/api/resp/builder.go

Index: milo/api/resp/builder.go
diff --git a/milo/api/resp/builder.go b/milo/api/resp/builder.go
index 3b25e8fa721caa0c212caa99dcebe10bafbcd9ec..46a30fe42a90870a7e15c6534760eff561072731 100644
--- a/milo/api/resp/builder.go
+++ b/milo/api/resp/builder.go
@@ -63,6 +63,11 @@ type Builder struct {
 	// machines that can run in a builder.  It has no meaning in buildbucket or dm
 	// and is expected to be nil.
 	MachinePool *MachinePool
+
+	// PrevCursor is a cursor to the previous page.
+	PrevCursor string `json:",omitempty"`
+	// NextCursor is a cursor to the next page.
+	NextCursor string `json:",omitempty"`
 }
 
 // MachinePool represents the capacity and availability of a builder.