closes #42

Fix TLS and improve robustness of dynamic Fastly backends
Changes:

• Default port handling — Defaults to port 443 for HTTPS, 80 for HTTP when not specified. Backend target is always host:port.
• Full TLS configuration — Adds sni_hostname() and check_certificate() alongside enable_ssl() for proper certificate validation.
• Timeouts — Adds connect_timeout(1s), first_byte_timeout(15s), between_bytes_timeout(10s).
• Explicit NameInUse handling — Instead of a fragile retry, checks error message for "NameInUse"/"already in use" and reuses the existing backend. All other errors are properly propagated.
• Human-readable backend names — Changed from hash-based (edgezero-dynamic-{hash}) to readable (edgezero-dynamic-https_example_com_443).
• Case-insensitive scheme check — Uses eq_ignore_ascii_case("https") instead of exact match.
• Debug logging — Logs backend creation, SSL enablement, and reuse at debug level.
• Simpler return type — Returns Result<String, EdgeError> (backend name) instead of Result<Backend, _> since the caller only needs the name.