Add designated node id to content tree

This node id will be used to decide if the content tree content need to be download or not
on that particular edge node. On a clustered eve devices this value can be used to determine which node is reponsible
to download the content. This is very similar to the Volume message where we use designated node id to determine the
node responsible to create the Volume.

On a single node eve installations assumption is that controller will pass in the non nil value,
but the code should be able to handle the nil values gracefully.

Signed-off-by: Pramodh Pallapothu <pramodh@zededa.com>

proto/config/storage.proto

@@ -161,6 +161,10 @@ message ContentTree {
   // network failure, the download process can fallback to the next datastore
   // in the list.
   repeated string ds_ids_list = 11;
+
+  // To inform the edge-node if the device receiving this content tree is
+  // responsible for content tree download or not.
+  string designated_node_id = 12;
 }
 
 // The protocol that the task will use to access the Volume