Add serializer for SubscribeSection

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

Change-Id: I0333f8cf8582eb9d3028ea3f5b35120b43d209c4
diff --git a/proto/cache.proto b/proto/cache.proto
index c860818..7613c04 100644
--- a/proto/cache.proto
+++ b/proto/cache.proto
@@ -444,3 +444,11 @@
   string pattern = 2;
   bool is_regular_expression = 3;
 }
+
+// Serialized form of com.google.gerrit.common.data.SubscribeSection.
+// Next ID: 4
+message SubscribeSectionProto {
+  string project_name = 1;
+  repeated string multi_match_ref_specs = 2;
+  repeated string matching_ref_specs = 3;
+}