Support content templates for messaging,

[robkane1]

Howdy,

I realise that this may be wishful thinking as I am not sure if this package is maintained anymore and the PRs don;t seem to be considered.

However, is there any chance of adding support for sending messages with template_sid and content_variables? Currently these messages fail with a body required error. The message struct doesn't support these fields.

https://www.twilio.com/docs/content/send-templates-created-with-the-content-template-builder

Happy to look at a PR if it will help and it will get looked at. Would prefer not to fork the whole library or create a custom integration. Seems like this is a fairly central feature these days.

Many thanks :)

[danielberkompas]

@robkane1 I will accept PRs for this feature. I intend to do basic maintenance on this package going forward, relying on the community to submit feature patches.