Re-implement custom OAuth2ProxyCall in JavaScript
The OSMOAuthProxyCall implementation overriding Rest.OAuth2ProxyCall with a version of serialize_params including the XML payload is currently implemented in C.
I don't remember if there was some issue with this type of vfunc in GJS earlier (having "out parameters", char **).
But this in any case works now, so this can be re-implemented in JS so we can get rid of some C code.