fix: parse forwarded header lists

[immanuwell]

Handles comma-separated X-Forwarded-* values when trusted headers are on. Multi-hop proxies do this IRL, AWS ALB appends XFF by default, so yep this is reachable.

Repro:

1. set http.trust_x_forwarded_headers: true
2. send a request with X-Forwarded-Host: public.example, proxy.local, X-Forwarded-Proto: https, http, or X-Forwarded-For: 203.0.113.10, 10.0.0.2
3. before this, Warpgate used the whole list. proto could fail parsing and host/logs got messy, not great tbh
4. now it uses the first non-empty value and falls back when the trusted header is empty

Tests: cargo test -p warpgate-common-http; cargo clippy -p warpgate-common-http --all-targets -- -D warnings; cargo fmt --check --package warpgate-common-http

Related: #882, #921, #886

[Eugeny]

Thank you! @all-contributors please add @immanuwell for code

[allcontributors]

@Eugeny

I've put up a pull request to add @immanuwell! 🎉