Pkce.Challengeval make : Verifier.t -> tWill always be a S256 challenge https://www.rfc-editor.org/rfc/rfc7636#section-4.2
val of_string : transformation:transformation -> string -> tval to_code_challenge_and_method : t -> string * stringhttps://www.rfc-editor.org/rfc/rfc7636#section-4.3