Represents a Xero OAuth 2.0 token pair.
@type t() :: %Xero.Auth.Token{ access_token: String.t(), expires_at: DateTime.t(), id_token: String.t() | nil, refresh_token: String.t() | nil, scopes: [String.t()], token_type: String.t() }
@spec auth_header(t()) :: {String.t(), String.t()}
@spec from_response(map()) :: t()