Extension - Enable http2 adaptive_window in hyper

[huntharo]

Motivations

• This should improve large body echo tests

Acceptance Criteria

• Test throughput on large body echo tests as baseline
• Add adaptive_window and re-test throughput

Docs

https://docs.rs/hyper/latest/hyper/server/conn/http2/struct.Builder.html#method.adaptive_window

hatoo/oha#540

let mut builder = hyper::client::conn::http2::Builder::new(TokioExecutor::new());
        builder.adaptive_window(true);