[Snippet Generation] Add support for mime/plain text payloads in the snippet generator

[andrueastman]

The mail apis now have examples sending mails as mime payloads. Examples include

• https://docs.microsoft.com/en-us/graph/api/user-sendmail?view=graph-rest-1.0&tabs=http#example-4-send-a-new-message-using-mime-format
• https://docs.microsoft.com/en-us/graph/api/message-forward?view=graph-rest-1.0&tabs=http#example-2-forward-a-message-using-mime-content

Support for the snippet generator for this needs to be added for the docs to be enriched.
AB#9666