Add builder options to adjust HTTP2 stream and connection window size

[seanmonstar]

The corresponding methods already exist on the underlying h2 builders.

• client::Builder::http2_initial_stream_window_size
• client::Builder::http2_initial_connection_window_size
• client::conn::Builder::http2_initial_stream_window_size
• client::conn::Builder::http2_initial_connection_window_size
• server::Builder::http2_initial_stream_window_size
• server::Builder::http2_initial_connection_window_size
• server::conn::Http::http2_initial_stream_window_size
• server::conn::Http::http2_initial_connection_window_size