[Proposal] CSharp: Make `MessageParser.CreateTemplate()` method to public & Add `Any.TryUnpack(MessageDescriptor, out IMessage)` method

[aka-nse]

Hello.

background:

Protocol Buffers for C# supports powerful reflections, and they provide flexiblity for message handling.

But, except prebuilt parsers such as a JSON, creating new message instance dynamically requires troublesome and slow way.

var messageDescriptor = GetSomeMessageDescriptor();

var message = Activator.CreateInstance(messageDescriptor.ClrType) as SomeMessage;

And unpacking Any instance with MessageDescriptor is more difficult.

This is in contrast to type validation by Any.Is(MessageDescriptor) requires MessageDescriptor.

proposal:

1. make public MessageParser.CreateTemplate()
   Already MessageParser class has CreateTemplate() method to build default instance, but it is internal.
   I propose to change it into public directly, or implement public proxy method into MessageDescriptor.
   
   var messageDescriptor = GetSomeMessageDescriptor();
   IMessage message = messageDescriptor.CreateTemplate();
   
   
2. add Any.TryUnpack(MessageDescriptor, out IMessage) method
   I propose to provide new way to unpack `Any` without closed generic type argument.
   
   var any = GetSomeAny();
   var messageDescriptor = GetSomeMessageDescriptor();
   any.TryUnpack(messageDescriptor, out IMessage unpacked);

Thanks a lot.