feat(client): add trust_env proxy resolver foundation

[GefMar]

Summary

Adds the trust_env proxy resolver foundation for FogHTTP.

This PR makes trust_env=True accepted and resolves supported proxy/TLS environment settings once during client configuration. It does not implement actual HTTP proxy routing or HTTPS CONNECT yet; those remain follow-up transport tasks.

Changes

• Added internal proxy resolver package under foghttp/_client/proxy/.
• Added typed proxy models for proxy URLs, targets, decisions, sources, and rules.
• Implemented environment parsing for:
  • HTTP_PROXY / http_proxy
  • HTTPS_PROXY / https_proxy
  • ALL_PROXY / all_proxy
  • NO_PROXY / no_proxy
  • SSL_CERT_FILE / ssl_cert_file
• Ignored uppercase HTTP_PROXY in CGI-like environments when REQUEST_METHOD is set.
• Added focused NO_PROXY matching for wildcard, exact host, suffix, port, localhost, IPv4, and IPv6 rules.
• Mapped SSL_CERT_FILE to TLSConfig only when explicit tls= is not provided.
• Removed the obsolete trust_env argument from the Rust RawClient constructor boundary.
• Added proxy/trust_env documentation and updated limitations/TLS/request-builder wording.

Security / Lifecycle Notes

• Environment values are snapshotted at client config creation, not read on each request.
• Proxy credentials are redacted in repr/error-facing surfaces.
• SSL_CERT_DIR, TLS verification disabling, PAC/WPAD, SOCKS, HTTP proxy routing, and HTTPS CONNECT remain unsupported.
• Proxy decisions stay in Python control-plane until transport support is implemented.