client certificates can be optional

[lidakaml]

• If require_client_certs is set to true or is missing, everything works as before and client certs are required.
• If require_client_certs is set to false epp_tls_worker will not expect a client certificate, but if provided will verify it.

[vohmar]

Thank you @lidakaml! Code is reviewed and approved. We need to add tests to this though before it's ready to merge.

[lidakaml]

Thank you for your quick response!
Greetings from nic.lv

[maricavor]

@lidakaml Optional cert behavior should be controlled at the TLS listener via {fail_if_no_peer_cert, false}. Could you check new PR #74 and test it if your requirements are satisfied.