Added optional client certs flag

[maricavor]

• Added optional client certificate support to EPP TLS server behind a new config flag require_client_certs.
• Default behavior unchanged (client certs required). When disabled, client certs are optional but still verified if provided.
• Added tests covering both with/without client certificates under the optional mode.

Configuration
New app env: require_client_certs
Omitted or true: require client certificates (existing behavior).
false: client certificates optional; if presented, they are verified.