feature: few thoughts for other functionalities

Glad there is an official release for Terraform GitHub action, we've been doing this thing internally and it is a pain to create the wheel...

Few thoughts based on my use case,

1. Is it possible to support terraform enterprise? maybe just by allowing customers to provide the `host`? I believe the URL (`path` section) schema for TFC and TFE should be the same, it is just the `host` is different.
2. Is it possible to make `directory` optional? Terraform workspace should be able to know where the working directory is. This will also be very helpful when multiple workspaces (source code in different repos) need to be triggered in a certain order and monitor the progress from a central place. e.g. repo1 is the metadata for the infra, and few other repos are the consumer of the 1st repo, which need to be run (without code change) when there is an update of the 1st repo.
3. This might be too much, but return a list of workspaces based on tags/keyword/commit sha can be interesting. e.g. trigger a run for all workspaces with `production` tag or `project1` in name.