Add support for content labels on commit

Add commit options which allow for setting labels on commit.
Prevents potential race between garbage collector reading labels
after commit and labels getting set.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

api/services/content/v1/content.proto

@@ -269,6 +269,9 @@ message WriteContentRequest {
 	// If this is empty and the message is not a commit, a response will be
 	// returned with the current write state.
 	bytes data = 6;
+
+	// Labels are arbitrary data to set on commit.
+	map<string, string> labels  = 7;
 }
 
 // WriteContentResponse is returned on the culmination of a write call.