Make SASL max message length configurable

Make SASL max message length configurable

The proposed OAUTHBEARER SASL mechanism will need to allow larger
messages in the exchange, since tokens are sent directly by the
client.  Move this limit into the pg_be_sasl_mech struct so that
it can be changed per-mechanism.

Author: Jacob Champion <jacob.champion@enterprisedb.com>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://postgr.es/m/CAOYmi+nqX_5=Se0W0Ynrr55Fha3CMzwv_R9P3rkpHb=1kG7ZTQ@mail.gmail.com