This interface is meant to replace the following ones (in use by some providers):
- httpclient.UserAgentString() (e.g. AzureRM, Google)
- terraform.UserAgentString (e.g. OpenStack, ProfitBricks)
- terraform.VersionString (e.g. AWS, AzureStack, DigitalOcean, Kubernetes)
This also proposes the initial UA string to be set to
HashiCorp Terraform/X.Y.Z (+https://www.terraform.io)