Sign responses with MESSAGE-INTEGRITY about mongooseice

[Dzol]

AFAICT we do this.

• encode for STUN: https://github.com/esl/MongooseICE/blob/master/lib/mongooseice/stun.ex#L23
• encode for TURN: https://github.com/esl/MongooseICE/blob/master/lib/mongooseice/udp/worker.ex#L105
• decode: https://github.com/esl/MongooseICE/blob/master/lib/mongooseice/stun.ex#L21

There's the following too:

$ grep -r -i --color verified lib/
lib//mongooseice/auth.ex:         true                       <- params.verified? do
lib//mongooseice/auth.ex:      false -> # Not verified
$ grep -r -i --color signed lib/
##
lib//mongooseice/auth.ex:    signed? = params.signed?
lib//mongooseice/auth.ex:      nil when not signed? ->
lib//mongooseice/auth.ex:      _ when signed? ->

Looking at the Jerboa code, however, it seems like we don't support message integrity for the short-term credential mechanism. There are tests for the existing functionality.