support SSL_CERT_FILE overrides

[vlinkz]

The reqwest dependency uses rustls-tls which bundles Mozilla's root CAs via webpki-roots. This means the HTTP client ignores system certificate stores and environment variables like SSL_CERT_FILE, blocking me from connecting to endpoints using internal/private CAs (e.g. statsig-forward-proxy with platform-issued TLS certificates).

Switching to rustls-tls-native-roots makes reqwest use rustls-native-certs to load certificates from the system store at runtime, which respects SSL_CERT_FILE and SSL_CERT_DIR.

[sandermvanvliet-stack]

I've been working on this exact issue (hacky approach in #33) and was looking for a better approach.

I see you have rustls-native-roots in this PR but would like to suggest to also add rustls-tls-webpki-root so that the original behaviour is maintained as well for systems that don't have any trust stores.

[vlinkz]

Added!