Dali external ptree values not serialized to client outside of SDS contexts

Normally, property trees are serialized to clients via SDS, and external values (large blob values commited to disk) are lazy fetched on demand.

However, in some cases a ptree is serialized outside of the context of SDS, e.g. getFileTree which is called when a foreign file is read.
In that situation, the regular ptree serialize method is used, and the tree is deserialized as a plain ptree at the client.
There is a bug where external values are skipped during this process, and the client receives an empty stub instead.

This was noticed (reported as bug), when a DFS file lookup to a foreign Dali was being made, that had large ECL and _rtlType values, that had both been externalized. Neither were serialized correctly, and it caused the client fail to deserialize the type info correctly.