Skill bot should create eoc as an activity, as IEndOfConversation doesn't have a Value property

[JonathanFingold]

Sample information

1. Sample type: [samples]
2. Sample language: [dotnetcore]
3. Sample name: 80.skills-simple-bot-to-bot/EchoSkillBot (probably in sample 81, too)

Describe the bug

With the current code, need to jump through hoops to set the Value property on the end of conversation activity. This is an issue because we tell developers to use the Value property as return value when the skill ends.

To Reproduce

Steps to reproduce the behavior:

1. Edit the skill bot in the simple skill sample.
2. After creating the end of conversation activity (as written in the sample), try to assign to its value property.
3. See error (compiler warning, as there is no such property on the IEndOfConversation type).

Expected behavior

Can assign a value to the activity.

Additional context

BotBuilder-Samples/samples/csharp_dotnetcore/80.skills-simple-bot-to-bot/EchoSkillBot/Bots/EchoBot.cs

Lines 17 to 22 in 492bbc1

	// Send End of conversation at the end.
	var messageText = $"ending conversation from the skill...";
	await turnContext.SendActivityAsync(MessageFactory.Text(messageText, messageText, InputHints.IgnoringInput), cancellationToken);
	var endOfConversation = Activity.CreateEndOfConversationActivity();
	endOfConversation.Code = EndOfConversationCodes.CompletedSuccessfully;
	await turnContext.SendActivityAsync(endOfConversation, cancellationToken);

[JonathanFingold]

After talking to Srinaath and TJ, it looks like the end dialog action in the skill does generate an endOfConversation activity, and there is a way to set the generated activity value property. There doesn't seem to be a way to set the generated activity code property, however.