mod_proxy_ajp: Add "secret" parameter to proxy workers to implement legacy

AJP13 authentication.  PR 53098.

Proposed by: Dmitry A. Bakshaev <dab1818 gmail com>


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1738878 13f79535-47bb-0310-9956-ffa450edef68