Feature Request: Support for StartTLS protocols

This is a generic tracking issue based on [this earlier discussion](https://github.com/mholt/caddy-l4/issues/313#issuecomment-3520371891). The feature would allow for protocols that use StartTLS to be handled by Caddy instead of only forwarded via passthrough, with either Caddy proxying the negotiation if it could detect SNI (_would presumably require successfully upgrading to TLS_), or only handling StartTLS as an outbound connection the proxied service similar to HTTPS => HTTP. --- I assume this is relevant to the `caddy-l4` project, but it may also be suited as a separate plugin for L4 if not? I don't actually expect anyone to tackle this. I am just opening / documenting the feature more generically for users to subscribe to and express interest via 👍 (_instead of the various separate issues already on the topic_) Maybe someone will give it a go, or can chime in with advice on approach/suitability to anyone that is interested in implementing