Implement Terraform provider that uses/imports "repo mode" and "get cached image" functionality

[mafredri]

1. Set up a new repo (coder/terraform-provider-envbuilder) from https://github.com/hashicorp/terraform-provider-scaffolding-framework
   • Set up CI/testing
2. Implement the Terraform provider according to RFC
   • Create the public facing API (TF inputs/outputs)
   • Write documentation for using the provider
   • Implement "repo mode"
   • Implement "get cached image"
3. Publish the provider

(Pre-)Requirements:

• Implement "repo-mode" to use the repo as source of truth for devcontainer/Dockerfile envbuilder#218
• Refactor “get cached image” logic to a separate package in envbuilder #11

Part of Envbuilder Terraform Provider RFC.

[bpmct]

Just waiting on docs, we'll been in sprint!

[johnstcn]

Documentation:

• README: https://github.com/coder/terraform-provider-envbuilder/blob/main/README.md
• Provider docs: https://github.com/coder/terraform-provider-envbuilder/blob/main/examples/resources/envbuilder_cached_image/envbuilder_cached_image_resource.tf (also online)
• Example templates: docker, kubernetes