Activity.ChannelId supports subchannels.

[tracyboehrer]

This is for discussion

Fixes #160

Notes/Discussion

• This uses a ChannelId class for Activity.ChannelId
  • This is created by the AP Spec channelId + ProductInfo.Id.
  • When serialized, it will conform to AP spec, with expected ProductInfo Entity if needed.
  • ChannelId.ToString() produces: {channelId}:{productInfo}
  • Passing ChannelId.Channel to ABS Rest API's
• TODO
  • Most likely need to supply ABS with the root channelId value (not the combined value).
  • ConversationReference has a string ChannelId property. What is the value? This needs some research.
  • Serialization tests
    • See ProductInfoTests for usage demo

Alternatives

• A class isn't the only solution, just more OO
• Activity.ChannelId could be the combined value, and we add in-memory Activity.Channel and Activity.SubChannel properties
• Activity.ChannelId is the combined value, and we have helpers to extract the components if needed
• Key behavior is that Activity arrives with expected channelId value and a ProductInfo Entity, which produces an in-memory Activity.ChannelId to satisfy AP expectations (a unique value). When serialized, this would be reversed.