Add a `plan` and `apply` subcommand

[skierkowski]

Background: Terraform Enterprise (Atlas) Legacy had integration with terraform open source for the command terraform push which packaged the local terraform configuration and pushed it to a workspace. Terraform Enterprise (Legacy and new) also both support integration for terraform plan and terraform apply but the configuration runs locally in the local context and only uploads the state after it is complete (without locking).

Goal: The goal of this feature is to provide the same user experience as terraform plan and terraform apply but the configuration gets executed in TFE not locally and also pushes the local configuration if provided.

Design: TBD