Is your feature request related to a problem? Please describe.
OAuthPrompt allows you to use a custom activity as the prompt by passing in PromptOptions.Prompt. But if you don't pass in an activity with a SigninCard or OAuthCard attachment, then OAuthPrompt injects its own card depending on the channel.

This causes issues if you want to prompt with a different type of card, such as an adaptive card. For example on Teams if I pass an activity with an adaptive card, OAuthPrompt also injects a SigninCard, resulting in a bad user experience.

Describe the solution you'd like
I want to be able to give OAuthPrompt any kind of activity/cards for prompting. Maybe a way to tell it "I'm providing my own prompt, don't mess with my attachments."

Describe alternatives you've considered

• Send the adaptive card without a sign-in button and let OAuthPrompt send just the sign-in button as a separate activity
  • Downside: Looks bad since it renders as 2 separate cards (in Teams anyway).
  • Downside: OAuthPrompt won't send the button if there's already a user token. We also have to figure out whether or not to send our adaptive card with an additional user token check.
• Write middleware to inspect outgoing activities and remove all SigninCard and OAuthCard (assuming we don't ever want to use those cards)
  • Downside: OAuthPrompt does unnecessary work to create its card, including (I think) HTTP calls
  • Downside: Additional code, kind of hack-y, etc.
• Derive a class from OAuthPrompt that overrides BeginDialogAsync to 1) inject a dummy SigninCard to prevent OAuthPrompt from doing extra work, and 2) set an OnSendActivities handler to remove the outgoing SigninCard, before calling the base BeginDialogAsync.
  • Downside: Additional code, kind of hack-y, etc.

[enhancement]