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

src/engine/engine_plugin.h

@@ -62,6 +62,15 @@ MJAPI void mj_loadPluginLibrary(const char* path);
 // scan a directory and load all dynamic libraries
 MJAPI void mj_loadAllPluginLibraries(const char* directory, mjfPluginLibraryLoadCallback callback);
 
+// registers a resource decoder
+MJAPI void mjp_registerDecoder(const mjpDecoder* decoder);
+
+// set default decoder definition
+MJAPI void mjp_defaultDecoder(mjpDecoder* decoder);
+
+// find a decoder that can process a given resource
+MJAPI const mjpDecoder* mjp_findDecoder(const mjResource* resource, const char* content_type);
+
 // =================================================================================================
 // MuJoCo-internal functions beyond this point.
 // "Unsafe" suffix indicates that improper use of these functions may result in data races.