Support Reading Fabric Properties safely

[cgettys-microsoft]

I would find it very useful to have a higher level wrapper over IFabricPropertyManagementClient
Covering at least:

• BeginGetProperty/EndGetProperty and/or BeginGetPropertyMetadata/EndGetPropertyMetadata
  And possibly also
• BeginEnumerateProperties/EndEnumerateProperties
  The other methods (e.g. all the put and Create and Delete and Name ones) are interesting in a general sense, but I don't personally have any immediate use case for them.

Ideally, I want to avoid a tokio dependency being required for these, but we'll see whether that makes any sense.
I'll attempt this myself, but thought I'd raise an issue to track it as well.

[youyuanwu]

Partially implemented in #163