feat: added tls handshake timeout

[lucastemb]

Adds an optional client-side TLS handshake timeout to TlsConnector.

The timeout is configured with with_handshake_timeout(Some(Duration)) and applies only while the TLS handshake is pending. If the handshake does not complete before the deadline, the connect future returns an io::ErrorKind::TimedOut. FallibleConnect preserves its existing behavior of returning the underlying IO on failure.

A regression test covers an idle peer timing out during handshake.

Motivated by:

• Allow configuring the TLS handshake timeout for HTTP clients. DataDog/saluki#178
• Acceptor timeout config #4 (comment)

[cpu]

Thanks! This looked broadly good, but I had some review feedback (see the PR description in #182). Ultimately instead of suggesting edits here I decided to open a separate PR with my feedback folded into your original commit so I could see what this looks like if we draw the rest of the owl and add timeout support to the server-side pieces as follow-up commits. I think it makes sense to look at the whole thing at once, but if folks disagree we can also take (some of || all of) the fixup commits I made in 182, apply them here, land this PR, and then look at the server-side separately.

[cpu]

With further reflection we've decided this doesn't need to live in the crate itself. Instead we've added more examples & documentation on how to impose timeouts externally: #187

Thanks for the PR, apologies it took so long to get to an outcome.