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/user/user_resource.h

@@ -46,6 +46,10 @@ MJAPI void mju_getResourceDir(mjResource* resource, const char** dir, int* ndir)
 // return < 0 if the resource is older than the given timestamp
 MJAPI int mju_isModifiedResource(const mjResource* resource, const char* timestamp);
 
+// given a resource, find its decoder and return the decoded spec
+// the caller takes ownership of the spec and is responsible for cleaning it up
+MJAPI mjSpec* mju_decodeResource(mjResource* resource, const char* content_type);
+
 #ifdef __cplusplus
 }
 #endif