Add serializer for Address

This commit adds a serializer for the Address 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: I37192013b1da23499fd6b64d012ea82696b74df4
diff --git a/proto/cache.proto b/proto/cache.proto
index 9ba6911..40f918b 100644
--- a/proto/cache.proto
+++ b/proto/cache.proto
@@ -388,3 +388,10 @@
   repeated string exclude_regular_expressions = 6;
   repeated string match_regular_expressions = 7;
 }
+
+// Serialized form of com.google.gerrit.entities.Address.
+// Next ID: 3
+message AddressProto {
+  string name = 1;
+  string email = 2;
+}