endpoint RequestClient
{
    RequestProgress(i32 request_id, Optional<u32> total_size, u32 downloaded_size) =|
    RequestFinished(i32 request_id, bool success, u32 total_size) =|
    HeadersBecameAvailable(i32 request_id, IPC::Dictionary response_headers, Optional<u32> status_code) =|

    // Certificate requests
    CertificateRequested(i32 request_id) => ()
}