Add support for the transactional email triggered campaign endpoint.

[darthsteven]

This code adds support for the transactional email triggered campaign endpoint:

https://developer.dotmailer.com/docs/send-transactional-email-using-a-triggered-campaign

Essentially this is a simple PR that defines the data types required for this API call, and adds the single method to the resources class so that it's callable etc.

[darthsteven]

And this is related to #17, although that issue is for the general transactional API, not the triggered campaign one.