Allow configuring the TLS handshake timeout for HTTP clients.

[tobz]

Context

When building an HTTP client, we should be able to configure a timeout specifically for establishing the TLS handshake with the downstream target system. This is a simple assurance against extended delays trying to connect to unresponsive systems.

Notes

There's no easy way (currently) to do this, as the way hyper-rustls/tokio-rustls are composed has the actual underlying connect future, and the TLS handshake future, all wrapped up in one without any existing ability to configure a timeout.

We would likely need to upstream support for this to tokio-rustls, to even expose a way to configure such a timeout, and then also to hyper-rustls to expose that configuration method in its own HttpsConnectorBuilder.