Add user agent for azure so the azure cli can pick it up about terraform-mixin

[carolynvs]

Here's a search of user agent in the azure rm provider. AWS implements a customizable user agent so I have hope we can too! https://github.com/search?q=repo%3Ahashicorp%2Fterraform-provider-azurerm%20user%20agent&type=code

[carolynvs]

Sounds like we just need to set the standard azure user agent env var and the azure resource will pick it up, same for aws as well.

hashicorp/terraform-provider-azurerm#2393

We can have the terraform mixin inject these env vars (like the az and aws mixins do today, but with opt-out) when their respective cloud resource providers are used.