sasl: Implement SASL authorisation identity via CURLOPT_SASL_AUTHZID

sasl: Implement SASL authorisation identity via CURLOPT_SASL_AUTHZID

Added the ability for the calling program to specify the authorisation
identity (authzid), the identity to act as, in addition to the
authentication identity (authcid) and password when using SASL PLAIN
authentication.

Fixes #3653
Closes #3790

NOTE: This commit was cherry-picked and is part of a series of commits
that added the authzid feature for upcoming 7.66.0. The series was
temporarily reverted in db8ec1f so that it would not ship in a 7.65.x
patch release.

Closes https://github.com/curl/curl/pull/4186