RelayCellBody: support other RelayCellFormats

In particular, the new format proposed in prop340 has the recognized and digest fields at different offsets. Additionally, the digest field is a different length.

Note that we can't embed the RelayCellFormat into the RelayCellBody itself, because the same body will be processed through different crypto layers, which can use different formats. Therefore we instead need to pass the format through its individual method parameters.

Progress on #1067