GstCustomMeta: simplify API
Move the GstStructure field into public struct for direct access, that's easier than having to call a function to get it. It is not an API/ABI breakage to extend the public structure of a GstMeta because they are always allocated by inside GStreamer. The structure is exposed already by gst_custom_meta_get_structure() which does not return a copy/ref, so it is locked into holding a GstStructure forever anyway.
Also add gst_meta_register_custom_simple() because most of the time only a name is required, tags and transform functions are more niche use-case.