Add support to add custom end-effector attachments for Panda, UR5, and Fetch

[zkingston]

This PR adds the functionality for custom end-effector attachments for the Panda, UR5, and Fetch robots. Attachments are specified as collections of spheres at a fixed offset from the end-effector of the robot. Both this offset and the collection of spheres can be set at runtime.

An example script (scripts/attachments.py) is provided to demonstrate how to create an attachment, attach a colliding sphere, and plan while avoiding collisions between the attachment and the environment.

[wbthomason]

Looks like there's a build failure with some of the use of std::optional on old GCC? I'll look into it in more depth tomorrow.