Move cncd/pipeline/pipeline/ to pipeline/ (#347)

* Refactor: move cncd/pipeline/ to pipeline/

* Refactor: move pipeline/pipeline/ to pipeline/

pipeline/backend/backend.go

@@ -0,0 +1,24 @@
+package backend
+
+import (
+	"context"
+	"io"
+)
+
+// Engine defines a container orchestration backend and is used
+// to create and manage container resources.
+type Engine interface {
+	// Setup the pipeline environment.
+	Setup(context.Context, *Config) error
+	// Start the pipeline step.
+	Exec(context.Context, *Step) error
+	// Kill the pipeline step.
+	Kill(context.Context, *Step) error
+	// Wait for the pipeline step to complete and returns
+	// the completion results.
+	Wait(context.Context, *Step) (*State, error)
+	// Tail the pipeline step logs.
+	Tail(context.Context, *Step) (io.ReadCloser, error)
+	// Destroy the pipeline environment.
+	Destroy(context.Context, *Config) error
+}