demo.launch.pyにuse_sim_timeを追加

[YusukeKato]

What does this implement/fix?

Gazebo以外のシミュレータにも対応できるようにdemo.launch.pyにuse_sim_timeオプションを追加します。

Does this close any currently open issues?

しません。

How has this been tested?

use_sim_timeがtrueになることを確認しました。

$ ros2 launch sciurus17_examples demo.launch.py use_sim_time:=true

$ ros2 param get /controller_manager use_sim_time
Boolean value is: True

Any other comments?

ありません。

Checklists

• I have read the CONTRIBUTING guidelines.
  • If there is no guideline in the repository, rt-net organization's guideline applies.
• I have checked to ensure there aren't other open Pull Requests for the same change.

[Kuwamai]

LGTMです