libpq: Trace frontend authentication challenges

libpq: Trace frontend authentication challenges

If tracing was enabled during connection startup, these messages would
previously be listed in the trace output as something like this:

F	54	Unknown message: 70
mismatched message length: consumed 4, expected 54

With this commit their type and contents are now correctly listed:

F	36	StartupMessage	 3 0 "user" "foo" "database" "alvherre"
F	54	SASLInitialResponse	 "SCRAM-SHA-256" 32 'n,,n=,r=nq5zEPR/VREHEpOAZzH8Rujm'
F	108	SASLResponse	 'c=biws,r=nq5zEPR/VREHEpOAZzH8RujmVtWZDQ8glcrvy9OMNw7ZqFUn,p=BBwAKe0WjSvigB6RsmmArAC+hwucLeuwJrR5C/HQD5M='

Author: Jelte Fennema-Nio <postgres@jeltef.nl>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/CAGECzQSoPHtZ4xe0raJ6FYSEiPPS+YWXBhOGo+Y1YecLgknF3g@mail.gmail.com