Add mTLS (Client Certificate) authentication for Gotify

[P4SQL]

I'd like to secure my internal infrastructure using a zero-trust model. My notification gateway (Gotify) is situated behind a reverse proxy (Traefik) that is capable to enforce mutual TLS (mTLS) using RequireAndVerifyClientCert for all API POST requests.

While DIUN excellently supports verifying custom CAs via tlsCaCertFiles, the Gotify notifier currently lacks the ability to present a client certificate and private key during the TLS handshake. As a result, DIUN cannot push notifications to mTLS-protected endpoints, resulting in 400 Bad Request or TLS handshake failures.

Please add configuration options to the Gotify notifier (and potentially the global HTTP client or other webhooks) to allow specifying a client certificate and private key :-)

Example configuration implementation:

notif:
  gotify:
    endpoint: https://go.example.com
    token: "..."
    tlsClientCertFile: /etc/ssl/certs/client.crt
    tlsClientKeyFile: /etc/ssl/private/client.key

Adding this feature would make DIUN highly adaptable for advanced homelab environments and enterprise utilizing internal PKI for strict service-to-service authentication.

Thank you.

Cheers
P4SQL