Accept and transmit padding in introduce2 plaintexts
According to rend-spec, we intentionally accept and discard extra bytes here. Doing so allows us to hide the size of our messages.
Closes #1031 (closed); see discussion there.
I think this code is ready, but I'm marking the MR as Draft
until all the discussions on #1031 (closed) and on torspec#222 (closed) are resolved