kldap: add support for TLS CA certificate handling

Summary:
This implements basic support for setting the TLS certificate request option (LDAP_OPT_X_TLS_REQUIRE_CERT) as well as a dedicated certificate
file for checking server certificate (LDAP_OPT_X_TLS_CACERTFILE).

Test Plan:
TLS connection to LDAP server with self-signed certificate

• scenario a) none of the new options set -> connection fails due to missing certificate to check against
• scenario b) use LdapServer::TLSReqCertNever which makes the connection succeed
• scenario c) use LdapServer::TLSReqCertHard and set tlsCACertFile which makes the connection succeed

Reviewers: mlaurent, dvratil

Reviewed By: mlaurent, dvratil

Subscribers: dvratil, KDE PIM

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D11520