Support originating audience for skills conversations

[EricDahlvang]

Fixes #798
Fixes #799
Fixes #800
Fixes #553

ConversationIdFactoryBase is backwards compatible, and will work with legacy implementations as well as those which support SkillConversationIdFactoryOptions (see attached .zip for a legacy implementation, and new implementations of SkillConversationIdFactory and SkillHttpClient)

Incidentally, I noticed the Python sdk does not have an implement of SkillHttpClient, while javascript and dotnet do. Added a comment to #553 regarding this.

skill_factories_and_clients.zip

[tracyboehrer]

1. I can't reliably attest that these are the appropriate Skills changes so I just compared against C#
2. Kudos on the ConversationIdFactoryBase method overload handling. This is pretty much how Axel recommended handling this. Use a Union or optional args and document in comments.
3. C# has the SkillConversationIdFactory in each bot (sample). It's in BotBuilder.core in Python. I don't understand the arguments for one vs the other. Just a noted difference. Though I see why you did it.
4. SkillConversationIdFactoryOptions: Axel has commented before on using @Property instead of normal class member vars. If your intention was to make them read-only (which doesn't really exist in Python since the caller can use the "protected" var) then OK. But it doesn't appear that C# provides read-only members. So I would just go with normal Python fields. self.from_bot_auth_scope, et.c..
5. SkillConversationReference: Same as Sample code snippets #4

[EricDahlvang]

Thank you @tracyboehrer ! I'll address these things, and tests, today. Before merging this, we should also determine what we are going to do about the SkillConversationIdFactory and missing SkillHttpClient. I'll bring this up in standup today.