Add a new plugin / extension mechanism called a resource provider along with retrofitting VFS on top of it.

A resource provider provides a mechanism for MuJoCo to read from filesystems other than the OS filesystem or the Virtual File System (VFS).

PiperOrigin-RevId: 525394983
Change-Id: I077ff5a7e2e76806b48b6defb531280aadc8b169

src/engine/CMakeLists.txt

@@ -33,8 +33,6 @@ set(MUJOCO_ENGINE_SRCS
     engine_derivative.h
     engine_derivative_fd.c
     engine_derivative_fd.h
-    engine_file.c
-    engine_file.h
     engine_forward.c
     engine_forward.h
     engine_inverse.c
@@ -50,6 +48,8 @@ set(MUJOCO_ENGINE_SRCS
     engine_print.h
     engine_ray.c
     engine_ray.h
+    engine_resource.c
+    engine_resource.h
     engine_sensor.c
     engine_sensor.h
     engine_setconst.c