Allow sending to specific template version

[rory-wilson]

In addition to specifying the template id, it would also be useful if the send an email / send an SMS endpoints and clients supported an optional template version number so that new versions of templates could be authored and used alongside older versions, especially when parameters change.

This would help with supporting templates across environments.

e.g.
POST /v2/notifications/email { "email_address": "sender@something.com", "template_id": "f33517ff-2a88-4f6e-b855-c550268ce08a", "template_version": 2, }