pgsql: Explicitly require password for SCRAM exchange

Explicitly require password for SCRAM exchange

This refactors the SASL init flow to set password_needed on the two
SCRAM exchanges currently supported. The code already required this
but was set up in such a way that all SASL exchanges required using
a password, a restriction which may not hold for all exchanges (the
example at hand being the proposed OAuthbearer exchange).

This was extracted from a larger patchset to introduce OAuthBearer
authentication and authorization.

Author: Jacob Champion <jacob(dot)champion(at)enterprisedb(dot)com>
Discussion: https://postgr.es/m/d1b467a78e0e36ed85a09adf979d04cf124a9d4b.camel@vmware.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/adcdb2c8dda4fb28e373ddff41a0de6936772249

Modified Files
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src/interfaces/libpq/fe-auth.c | 28 +++++++++++++++-------------
1 file changed, 15 insertions(+), 13 deletions(-)