feat: mTLS configuration for HTTPS and TCP monitors

[cruscio]

Please add the ability to specify client certificate & key, and server CA certificate, for monitoring of HTTPS & TLS endpoints that require mTLS handshakes (and/or use self-signed server certificates). For example: monitoring Kafka Broker listener endpoints.

A couple similar examples:

• https://github.com/louislam/uptime-kuma/issues/139
• https://github.com/statping/statping/issues/597

[luciofsl]

@TwiN any update for this feature? It would be great to have this working...

[TwiN]

Sounds like an excellent feature to add in https://github.com/TwiN/gatus#client-configuration