connect: Expose more keepalive options for HttpConnector

[LucioFranco]

Expose more options from https://docs.rs/socket2/0.4.2/socket2/struct.TcpKeepalive.html for HttpConnector.

[seanmonstar]

This is probably fine. The biggest question is whether to expose the methods universally, or whether to add cfg attributes on them.

[LucioFranco]

I think maybe deprecate the current method and expose a new config based one similar to what socket2 exposes?

[charliegracie]

Is there any update on this? A roadmap item?