Add server mode to provide HTTP REST interface for state backend

[binlab]

Thanks for your work! Can I ask you to add some features? As I see you currently support encrypt/decrypt via Vault but will be nice to add functionality to work Terrahelp in server mode for transit encryption and provide HTTP REST interface for Terraform https://www.terraform.io/docs/backends/types/http.html In this case, we can run Terrahelp like local service and make transit encryption and storing the result to S3, Terraform Cloud or other backends. As a Vault will do transit encryption we can even do collaborations and prevent store any keys on the local environment