sasl: Allow backend mechanisms to "abandon" exchanges

sasl: Allow backend mechanisms to "abandon" exchanges

Introduce PG_SASL_EXCHANGE_ABANDONED, which allows CheckSASLAuth to
suppress the failing log entry for any SASL exchange that isn't actually
an authentication attempt. This is desirable for OAUTHBEARER's discovery
exchanges (and a subsequent commit will make use of it there).

This might have some overlap in the future with in-band aborts for SASL
exchanges, but it's intentionally not named _ABORTED to avoid confusion.
(We don't currently support clientside aborts in our SASL profile.)

Adapted from a patch by Zsolt Parragi.

Author: Zsolt Parragi <zsolt.parragi@percona.com>
Co-authored-by: Jacob Champion <jacob.champion@enterprisedb.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://postgr.es/m/CAN4CZFPim7hUiyb7daNKQPSZ8CvQRBGkVhbvED7yZi8VktSn4Q%40mail.gmail.com