Add support for Cartesian paths

[Schnilz]

Hi,
thank you for writing this, especially with the good examples :)
I missed the cartesian planner, so i added it here.
I don't know why the moveit2 interface is so weird, but the code works (and calls the same service as the GUI in rviz does i think).

closes #6

[AndrejOrsula]

Hello @Schnilz,
Great! Thank you for your contribution.
I will test, review and merge it in the coming days. 😄

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I don't know why the moveit2 interface is so weird, but the code works (and calls the same service as the GUI in rviz does i think).

I have to agree that the generic ROS 2 interface of MoveIt 2 can be a bit difficult to follow. Maybe there is a much simpler way, but I have not found it yet. The code in this repo can be quite ugly, especially when trying to reduce the number of memory allocation calls and create only clients that are actually used.

I also noticed a difference in performance between services and actions that achieve the same goal. That's why both options are currently available, despite adding yet another level of ugliness to the code.

[AndrejOrsula]

Thank you again for this PR! It works and looks good to me. 😄 I added some cosmetic comments.

Could you run the pre-commit hooks (it will fix formatting and sort imports)? There is a small setup script under .git_hooks/setup.bash that would install it on your system and setup the repository. If not, I will run them before merge.