Add [SdfLib](https://github.com/UPC-ViRVIG/SdfLib) plugin to first-party SDFs.

The following dependencies are disabled with a cmake patch file.
- Enoki
- OpenMP
- Assimp
- FCPW

PiperOrigin-RevId: 565352386
Change-Id: Icff9de68408e0afa2032bb83b24491fc4e41d181

src/user/user_mesh.cc

@@ -394,7 +394,7 @@ void mjCMesh::Compile(const mjVFS* vfs) {
   }
 
   // create using marching cubes
-  if (is_plugin) {
+  else if (is_plugin) {
     LoadSDF();
   }
 

src/user/user_util.cc

@@ -669,6 +669,8 @@ std::string mjuu_extToContentType(std::string_view filename) {
     return "model/stl";
   } else if (!strcasecmp(ext.c_str(), ".obj")) {
     return "model/obj";
+  } else if (!strcasecmp(ext.c_str(), ".ply")) {
+    return "model/ply";
   } else if (!strcasecmp(ext.c_str(), ".msh")) {
     return "model/vnd.mujoco.msh";
   } else if (!strcasecmp(ext.c_str(), ".png")) {