Add serializer for LabelValue

This commit adds a serializer for the LabelValue entitiy.
The eventual goal is that we serialize CachedProjectConfig. The
entity is too large to be serialized directly, though, so we
divide and conquer.

This commit moves the AutoValue representation to the entities
package to allow the serializer packages to keep its dependencies
minimal.

Change-Id: If04e7efb06cb8c53b995296e39194556a3d97dab
diff --git a/proto/cache.proto b/proto/cache.proto
index c80afef..426c8eb 100644
--- a/proto/cache.proto
+++ b/proto/cache.proto
@@ -406,3 +406,10 @@
   repeated GroupReferenceProto groups = 5;
   repeated AddressProto addresses = 6;
 }
+
+// Serialized form of com.google.gerrit.entities.LabelValue.
+// Next ID: 3
+message LabelValueProto {
+  string text = 1;
+  int32 value = 2;
+}