feat: Add Terraform Provider for agent resources (#368)

* ci: Update DataDog GitHub branch to fallback to GITHUB_REF

This was detecting branches, but not our "main" branch before.
Hopefully this fixes it!

* Add basic Terraform Provider

* Rename post files to upload

* Add tests for resources

* Skip instance identity test

* Add tests for ensuring agent get's passed through properly

* Fix linting errors

* Add echo path

* Fix agent authentication

* Update codersdk/files.go

Co-authored-by: Bryan <bryan@coder.com>

Co-authored-by: Bryan <bryan@coder.com>

cmd/terraform-provider-coder/main.go

@@ -0,0 +1,13 @@
+package main
+
+import (
+	"github.com/hashicorp/terraform-plugin-sdk/v2/plugin"
+
+	"github.com/coder/coder/provisioner/terraform/provider"
+)
+
+func main() {
+	plugin.Serve(&plugin.ServeOpts{
+		ProviderFunc: provider.New,
+	})
+}