Create mjpDecoder, mj_parse, and mju_decodeResource

- plugin system similar to mjpResourceProvider, but instead of loading a resource it converts an existing mjResource into an mjSpec.
  - The returned spec is then composed into the referencing spec.
  - This enables different file types to generate arbitrary specs, and allows us to separate format parsing from compilation code.

Follow up CLs will move some of the logic in src/engine for PNG, USD, KTX, OBJ loading into decoders.

The mj_parse function MjSpec from a given file, it's a more generic version of mj_parseXML.

In it's implementation, mj_parse as opposed to mj_parseXML will look for any registered decoder and
not assume we are striclty dealing with MJCF.

PiperOrigin-RevId: 826149497
Change-Id: I0ece26904280cb94bd5ded6dd5a565c539d60254

python/mujoco/specs.cc

@@ -281,8 +281,8 @@ PYBIND11_MODULE(_specs, m) {
           spec = LoadSpecFileImpl(
               filename, files,
               [&error](const char* filename, const mjVFS* vfs) {
-                return InterceptMjErrors(mj_parseXML)(
-                    filename, vfs, error, sizeof(error));
+                return InterceptMjErrors(mj_parse)(
+                    filename, nullptr, vfs, error, sizeof(error));
               });
           if (!spec) {
             throw py::value_error(error);