[FEATURE] set USE_PROXY_PROTOCOL multisite compatible

### What's needed and why?

My bunkerweb configuration is fully multisite with lots of reverse proxy. I would like to add stalwart as a new service which use proxy_protocol for it's configuration. [https://stalw.art/docs/server/reverse-proxy/nginx](https://stalw.art/docs/server/reverse-proxy/nginx)
USE_PROXY_PROTOCOL is only available globally, I cannot enable USE_PROXY_PROTOCOL only for stalwart and leave all my other service with classic http reverse proxy (without proxy protocol)

### Implementations ideas (optional)

_No response_

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct

Start by tracing how USE_PROXY_PROTOCOL is read and propagated in the multisite reverse-proxy configuration; the payload names no source files or tests. Use the Stalwart reverse-proxy guidance linked in the issue as the behavior reference, and verify that one service can use proxy protocol while other services retain classic HTTP proxying.

Written by the indexing model from the issue text.