Concat Attachments if exist in Prompt::AppendChoices

[EricDahlvang]

Describe the bug

Attachments from Prompt message should combine with Attachments from Choice (if they are both present).

To Reproduce

Steps to reproduce the behavior:

1. Create a ChoicePrompt with a Prompt message containing an attachment and some choices
2. set PromptOptions.Style to ListStyle.HeroCard
3. Message the bot
4. Notice the attachment on the Prompt message is not shown

Code:

https://github.com/microsoft/botbuilder-python/blob/master/libraries/botbuilder-dialogs/botbuilder/dialogs/prompts/prompt.py#L221

Expected behavior

Prompt message attachment should be combined with Choice Attachment (if both are present)

Additional context

Parity with cs: microsoft/botbuilder-dotnet#2520

[bug]