Use the terraform-plugin-sdk user agent string with running tf version

[awilliamsOM1]

Http requests now using the user-agent built by the terraform-plugin-sdk using the running terraform version string instead of the default Go user agent.

Before: Go-http-client/1.1
After: HashiCorp Terraform/0.12.25 (+https://www.terraform.io) Terraform Plugin SDK/1.6.0

Starting with Terraform version v.0.12 the running version Terraform can be obtained from the provider. However this field is not included in prior versions of Terraform and the running version cannot be discovered. As a way to support older versions, the version string of "0.11+compatible" is used. This was inspired by the way the aws provider plugin works, hashicorp/terraform-provider-aws@0e183c1

Fix for issue #276.

[mrparkers]

thanks for this, this is a good change. I made one small update to compute the user agent in the provider package, since I want to keep the keycloak package free of anything terraform related so it can be used outside of the context of this provider.

[awilliamsOM1]

Thanks for making the change and fixing the tests. Moving the user agent building to prevent a terraform dependency in the keycloak package makes sense. Thanks!