Mock components対応

[Kuwamai]

What does this implement/fix?

Mock componentsに対応し、実機不要で動作確認ができるようになります。

Does this close any currently open issues?

しません

How has this been tested?

READMEに記載の手順で起動し、joint_values、gripper_control等のサンプルや、RVizを用いてアームやグリッパが動くことを確認しました。
https://github.com/rt-net/crane_x7_ros/tree/feature/add_mock_components/crane_x7_examples#準備mock-componentsを使う場合

Any other comments?

こちらのPRマージ後にレビューお願いします
rt-net/crane_x7_description#12

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.