python bindings resource provider API

[guyazran]

The problem

I require a way to build a highly cluttered mujoco model whose assets are loaded dynamically to memory in python. This follows the discussion in issue #1054.

Suggested solution

The agreed solution in the original issue is to implement access to provide access to mjpResourceProvider through the python bindings.

Considered alternatives

• loading with mujoco.MjModel.from_xml_string(xml_string, assets_dict) is limited to 2,000 assets in the dictionary via the mjMAXVFS constant.
• saving the entire model and assets to disk and reloading is highly inefficient.
• using RAMDisk solutions is not multi-platform friendly in python without using third-party file systems.