Replacing line breaks with <br> in markdown text

[janusbal]

Description

Android, web and desktop support \n in markdown texts while iOS does not. This adds support to line breaks by replacing them with
html tag before generating the html string.

Sample Card

{
  "$schema": "http://adaptivecards.io/schemas/adaptive-card.json",
  "type": "AdaptiveCard",
  "version": "1.0",
  "body": [
 
    {
 
      "type": "Container",
 
      "items": [
 
        {
 
          "type": "TextBlock",
 
          "text": "Hello User",
 
          "wrap": true
 
        },
 
        {
 
          "type": "TextBlock",
 
          "text": "**bold**\n\n_cursive_",
 
          "wrap": true
 
        }
 
      ]
 
    }
 
  ]
}

How Verified

How you verified the fix, including one or all of the following:

1. New unit tests that were added if any. If none were added please add a quick line explaining why not.
2. Existing relevant unit/regression tests that you ran
3. Manual scenario verification if any; Do include .gif's or screenshots of the testing you performed here if you think that it
   will aid in code reviews or corresponding fixes on other platforms for eg.

Microsoft Reviewers: Open in CodeFlow

[jwoo-msft]

@janusbal

I think you forgot to run clang-format.

other than that, it looks good.