Feature Request: Support context data in serialization callbacks

[GoogleCodeExporter]

One feature of the standard .NET serializers is the ability to assign context 
information into StreamingContext objects which then exposes it to the 
serialization callbacks, such as [OnDeserializing] handlers.

This feature would be quite handy when porting code to protobuf-net.  The 
StreamingContext API itself isn't what's interesting here, just the ability to 
pass some context data to ProtoAfterDeserialization, for example.  That feature 
would save me some gross thread-static global messyness!

Original issue reported on code.google.com by cold...@gmail.com on 18 Jun 2011 at 2:58