Get the Terraform version being used

[wata727]

See also terraform-linters/tflint-ruleset-aws#859, terraform-linters/tflint-ruleset-terraform#156

For some rules, you may want to change behavior based on the version of Terraform being used. It would be nice to be able to easily get the Terraform version using the SDK.

The terraform_workspace_remote rule determines the version based on the required_version in the terraform blocks. One idea is to expose this functionality as an API in the SDK.

Other ideas would be to look for a .terraform-version file, or add terraform_version as a TFLint config (It used to exist but has now been removed terraform-linters/tflint#311) and reference that.