This change introduces MjVFS, an explicit object to mirror the C mjVFS. This is meant to replace the MjSpec.assets dict. This latter while convenient guides users towards harmful authoring patterns with respect to data duplication and spec attachment workflows. Making VFS management explicit should encourage better memory usage and allow us to make better compile time optimizations.
From this change, `spec.assets` is deprecated. However we will temporarily support backwards compatibility due to the wide spread usage. An error will be thrown if calling code tries to use a spec that uses both the assets dict and the new MjVfs.
PiperOrigin-RevId: 908772050
Change-Id: I77c6d0369307fc300c954768fed17401261e18da
Sam Haves