Build the upstream refresh-token-exchange form for a proxied /token
request carrying grant_type=refresh_token. Relays the client-supplied
refresh_token to the upstream token endpoint with the fixed upstream
credentials and (RFC 8707) resource, so a client that obtained a refresh
token via the proxy can refresh through it — matching the
refresh_token grant the proxy advertises in its AS metadata.
Build the upstream refresh-token-exchange form for a proxied /token request carrying grant_type=refresh_token. Relays the client-supplied refresh_token to the upstream token endpoint with the fixed upstream credentials and (RFC 8707) resource, so a client that obtained a refresh token via the proxy can refresh through it — matching the refresh_token grant the proxy advertises in its AS metadata.