Add args field to mjResource for decoder and encoder arguments.

Add an optional `const char* args` field to `mjResource` to allow passing
resource arguments/hints (such as requested channel count or encoding options)
to resource decoders and encoders.

PiperOrigin-RevId: 959774058
Change-Id: Icc41a2bb3895fef24f98c5fa9a77cdad092a6bc7

include/mujoco/mjplugin.h

@@ -30,6 +30,8 @@ typedef struct mjResource_ {
   mjVFS* vfs;                                   // pointer to the VFS
   char timestamp[512];                          // timestamp of the resource
   const struct mjpResourceProvider* provider;   // pointer to the provider
+  const char* args;  // resource arguments/hints, URI query format key=val&...
+                     // (optional)
 } mjResource;
 
 // callback for opening a resource, returns zero on failure.