Modify mj_PLUGIN_LIB_INIT to support multiple plugins in the same compilation unit.

Previously on MSVC we used DllMain to register plugins, you cannot have multiple definitions of DllMain in a single unit so you would get errors if you tried to register two plugins. This modifies the implementation to insert a function pointer into the C runtime initialization instead.

PiperOrigin-RevId: 896612678
Change-Id: I07732147b955d741c836acff6da986db0e9b9eff

plugin/obj_decoder/obj_decoder.cc

@@ -114,7 +114,7 @@ int CanDecode(const mjResource* resource) {
 
 }  // namespace
 
-mjPLUGIN_LIB_INIT {
+mjPLUGIN_LIB_INIT(obj_decoder) {
   mjpDecoder decoder;
   mjp_defaultDecoder(&decoder);
   decoder.content_type = "model/obj";