Have Lox credential return to the client a formated bridge line

I see in my open invitation bridge line the following:

'{"addr":[byte array],"port":YYYY,"uid_fingerprint":XXXXXXXXXXXXXXXXXXXX,"info":[byte array]}'

IIRC, the info field is the PT-specific params. It might also be nice for the byte array values to be easier to be converted into strings before we're dealing with it in javascript so that it's easier to turn into a bridge line that Tor Browser will understand.

This isn't urgent, because we are only requesting obfs4 bridge from rdsys for now, but in the future we may want to distribute other transports over lox.