Added new Microsoft Teams notifier

[mberinger]

This cloud build notifier effectively extends the HTTP notifier and provides the notification using a similar format to that of the Slack notifier. An example of a successful, failed and cancelled build notification is shown below:

[LOZORD]

Hi Mike, thanks for your PR! We're considering creating a cloud-build-notifiers-community repo for community-maintained notifiers but we would like to see more interest and PRs (like this one) before doing so. In the meantime, you are welcome to use a fork (these notifiers are just wrappers around the GCB Pub/Sub - no secret sauce yet), and please keep the feedback coming our way.