pgsql: 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(dot)parragi(at)percona(dot)com>
Co-authored-by: Jacob Champion <jacob(dot)champion(at)enterprisedb(dot)com>
Reviewed-by: Chao Li <li(dot)evan(dot)chao(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAN4CZFPim7hUiyb7daNKQPSZ8CvQRBGkVhbvED7yZi8VktSn4Q%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/c4ff16339f07d1e253bdf18e5da5fa25f62a750d

Modified Files
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src/backend/libpq/auth-sasl.c | 24 ++++++++++++++++++++++--
src/backend/libpq/auth.c | 32 +++++++++++++++++++++++++-------
src/include/libpq/sasl.h | 15 +++++++++------
3 files changed, 56 insertions(+), 15 deletions(-)