Markdown: External link icon did not show up in Adaptive Cards

[compulim]

Screenshots

When typing markdown, the external link icon shows up.

When typing card markdown, the external link icon did not show up.

Version

4.13.0

Describe the bug

The new "external link icon" only works with the command markdown, but not the card markdown command.

Steps to reproduce

1. Go to https://compulim.github.io/webchat-loader/
2. Type card markdown

Expected behavior

The links in the Adaptive Cards should show an external link icon.

Additional context

This is because the CSS class name was not applied to the content of Adaptive Cards, but only the TextContent.js.

Action items

• Update the Styles/StyleSet/AdaptiveCardRenderer.ts and use the external link icon, as like in Styles/StyleSet/TextContent.ts
• Make sure our existing test cover the external link icon, otherwise, add new tests

[Bug]