Add pluggable resource writing to MuJoCo

Extend mjpResourceProvider with an optional write callback (write)
so that mj_encode, mj_saveXML, and mj_saveModel can write to any
registered provider.

PiperOrigin-RevId: 945202741
Change-Id: I37903425260932e555f4a8c2392c4ff8c2e6cc06

include/mujoco/mujoco.h

@@ -1588,6 +1588,11 @@ MJAPI void mju_closeResource(mjResource* resource);
 // return negative value if error.
 MJAPI int mju_readResource(mjResource* resource, const void** buffer);
 
+// Write resource data via its resource provider, return bytes written or -1 on error.
+// Nullable: vfs, error
+MJAPI mjtSize mju_writeResource(const char* name, const void* buffer, mjtSize nbytes,
+                                const mjVFS* vfs, char* error, size_t nerror);
+
 // For a resource with a name partitioned as {dir}{filename}, get the dir and ndir pointers.
 MJAPI void mju_getResourceDir(mjResource* resource, const char** dir, int* ndir);