mod_proxy.xml: add vhost/server contextes for ProxySet

While reading PR 62422 I noticed that ProxySet's available
contextes list only 'directory', that looks odd:

AP_INIT_RAW_ARGS("ProxySet", set_proxy_param, NULL, RSRC_CONF|ACCESS_CONF



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1834524 13f79535-47bb-0310-9956-ffa450edef68