[PORT] Add originatingAudience to SkillHttpClient.PostActivityAsync

[tomlm]

Port this change from botbuilder-dotnet/master branch:
microsoft/botbuilder-dotnet#3477

Fixes #3449
Fixes #3450
Fixes #3451

Changed projects

• Microsoft.Bot.Builder
• integration
• Microsoft.Bot.Builder.TestProtocol
• Microsoft.Bot.Builder.Tests

[EricDahlvang]

closing since these issues already exist in the javascript repo