[FEATURE] Support markdown within Microsoft Teams messages

**Is your feature request related to a problem? Please describe.**
As a user, I'd like to be able to use markdown to send messages using markdown. Currently, Microsoft Graph API does not specifically support markdown, but it accepts HTML. See some discussion about this [here](https://discuss.rapid7.com/t/microsoft-teams-supports-markdown/693).

The following markdown is supported within Microsoft Teams: https://support.microsoft.com/en-us/office/use-markdown-formatting-in-microsoft-teams-4d10bd65-55e2-4b2d-a1f3-2bebdcd2c772#ID0EBD=Desktop

This request is to be able to create a generalized function to convert markdown to HTML (for use across apps where this is relevant), and to implement this in Microsoft Teams using a new called Markdown option under `Content Type`. We would do the conversion, and send it as HTML. We can also consider using an existing library e.g. https://github.com/showdownjs/showdown for this as well.