libcontainer: provide an option to disable `saveStates`

In libcontainer/container_linux.go there is a function called saveStates() which will write a state.json file to the filesystem. The function is called by updateState() so every time a container is run the file will be generated, though it may be not mandatory in some use cases.

Although libcontainer seems to be only intended for runc, an OCI runtime implementation, I still wonder if there are any posibilities that it can provide an option for those who only uses libcontainer as part of their programs to disable the state.json file writing behavior?

func (c *Container) saveState(s *State) (retErr error) {
	tmpFile, err := os.CreateTemp(c.root, "state-")
	if err != nil {
		return err
	}

	defer func() {
		if retErr != nil {
			tmpFile.Close()
			os.Remove(tmpFile.Name())
		}
	}()

	err = utils.WriteJSON(tmpFile, s)
	if err != nil {
		return err
	}
	err = tmpFile.Close()
	if err != nil {
		return err
	}

	stateFilePath := filepath.Join(c.root, stateFilename)
	return os.Rename(tmpFile.Name(), stateFilePath)
}

Start by reading libcontainer/container_linux.go, especially updateState() and saveState(), to understand how state.json is written during container runs. Determine an appropriate option for callers that use libcontainer directly, then verify that disabling the behavior avoids the file write without changing the default behavior.