feat: include loaders for public robot descriptions

[julianmueller]

Is your feature request related to a problem?

I just stumbled upon https://github.com/robot-descriptions/robot_descriptions.py which offers a variety of robot_descriptions. Maybe we can incorporate them as somewhat of a plugin? Not to add as a requirement, but as an optional urdf/etc loader, from an "official" curated list.

Describe the solution you'd like

Its basically just a wrapper and parser for the loader from https://github.com/robot-descriptions/robot_descriptions.py

from linkforge.core import example_loader
// this links to `from robot_descriptions.loaders.pinocchio import load_robot_description`

rob1 = examples_loader.load_robot("upkie_description")
// this links to `load_robot_description("upkie_description")`

Describe alternatives you've considered

No response

Additional context

This is of low priority.

[arounamounchili]

Hey @julianmueller, nice find! robot_descriptions.py look like a solid library,

Before we go further, there's one constraint we need you to think through: linkforge-core has a strict Zero-Dependency policy. Right now we have exactly one runtime dep, pyyaml, and we'd like to keep it that way. So the question is, can you make this work without touching [project.dependencies] in core/pyproject.toml?

If you can put forward a proposal that meets this need, I am open to discussing it. I believe that the Python packaging actually has a built-in mechanism for handling this kind of optional functionality, which is worth looking into. This could therefore be implemented once we have finished version 1.4.0, provided it is possible to do so whilst strictly adhering to the ‘zero dependency’ policy of linkforge-core.