curl: --sasl-authzid added to support CURLOPT_SASL_AUTHZID from the tool

Ref: https://github.com/curl/curl/issues/3653
Ref: https://github.com/curl/curl/pull/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

src/tool_help.c

@@ -391,6 +391,8 @@ static const struct helptxt helptext[] = {
    "Wait time between retries"},
   {"    --retry-max-time <seconds>",
    "Retry only within this period"},
+  {"    --sasl-authzid <identity> ",
+   "Use this identity to act as during SASL PLAIN authentication"},
   {"    --sasl-ir",
    "Enable initial response in SASL authentication"},
   {"    --service-name <name>",