`hostcalls::get_map` crashes Envoy when an HTTP header value is not a UTF-8 encoded string

[yskopets]

Summary

• HTTP spec allows arbitrary octets in header values
• However, Rust SDK is using String type that is limited to UTF-8 strings only
• hostcalls::get_map panics when an HTTP request contains a header with a value that is not a UTF-8 encoded string

[haikuginger]

Seconding this issue. Having get_http_request_headers return Result instead of being able to panic on non-UTF-8 input would be great.