container-commit: support --squash to squash layers into one

Allow users to commit containers into a single layer.

Usage
```bash
podman container commit --squash <name>
```

Signed-off-by: Aditya R <arajan@redhat.com>

pkg/domain/entities/containers.go

@@ -154,6 +154,7 @@ type CommitOptions struct {
 	Message        string
 	Pause          bool
 	Quiet          bool
+	Squash         bool
 	Writer         io.Writer
 }