Summary:
Per [[ https://github.com/yugabyte/yugabyte-db/issues/28108 | #28108 ]], this diff adds a gflag to enable SCRAM with channel binding (referred to
as SCRAM-SHA-256-PLUS during the SASL message exchange). The default value of the flag
`ysql_enable_scram_channel_binding` is false.

The aim is to make the option to use channel binding explicit as connection manager does not
support SCRAM with channel binding. Thus, clients forcing channel binding would see differing
behaviour with and without connection manager by default. Forcing explicit enabling of channel
binding ensures this is intentional.
Jira: DB-17738

Test Plan: Jenkins: all tests

Reviewers: skumar, mkumar, asrinivasan, arpit.saxena, jason

Reviewed By: arpit.saxena

Subscribers: jason, yql

Differential Revision: https://phorge.dev.yugabyte.com/D45834