@spec jwt(
  target :: Rivet.Ident.Factor.t(),
  hostname :: String.t(),
  testing_only_expiration_offset :: nil | integer()
) :: {:ok, token :: String.t(), claims :: map()}

Generate an auth JWT to our specification

iex> alias Rivet.Ident.Factor
iex> {:ok, token, claims} = jwt(%Factor{id: "AB", details: %{type: "acc"}}, "example.com", 5*60)
iex> claims.aud
"caa1:acc:example.com"
iex> claims.sub
"cas1:AB"
iex> String.slice(token, 0..1)
"ey"

iex> {:ok, token, claims} = jwt("narf", "example.com", 5*60)
iex> claims.aud
"caa1:acc:example.com"
iex> claims.sub
"cas1:narf"
iex> String.slice(token, 0..1)
"ey"

iex> {:ok, "ey" <> _rest, %{sub: "cas1:userid"}} = jwt("userid", "hostname")