More robust and modern extensions registration

[julien-blanchon]

Hello,

I feel like like extension need to bring a lot of boilerplate in order to patch holosoma package.

I nice way to solve this is to use python entry-points. And directly read the registred package from holosoma package and apply the right patching. I honestly feel like it will 10x the developers experience.

I've suggested and implemented a similar pattern on the mjlab project, it should give you enought context:

• Add plugin system with auto-subscription mujocolab/mjlab#245
• Add plugin system for external task registration. mujocolab/mjlab#282
• Gregwar/mjlab_cartpole@c0b6f8f