Support for MJCF Files

[jleibs]

MJCF is a common file format for kinematic robot (and other general physics simulation) descriptions.

It's part of the MuJoCo physics engine.

Although not as widely used as URDF it has some nice properties and shows up frequently in simulators:

• Genesis
• Nvidia Isaac

At a minimum we should have a reference dataloader implemented in Python or C++ which loads an MJCF project into Rerun by translating to a set of Transforms and Meshes.

However, coercing into the Rerun hierarchy and expanding transforms loses some of the fidelity that could be expressed. This could be a good opportunity to take a look at #1533 and think about some form of Viewer-derived joint-space -> Transform conversion leveraging the MJCF model.

In an ideal world a user would be able to publish a single component containing an array of joint-positions at each time stamp and have the robot model fully articulate in the correct way.