feat(client): Add Expect: 100-continue body wrapper

[bountis]

Add a request body wrapper which handles the Expect: 100-continue header flow.
https://www.rfc-editor.org/rfc/rfc9110.html#name-expect

Also adds a utility wrap which wraps a request body in ExpectContinueBody, adds the Expect: 100-continue header if not present, adds an on_informational hook which signals to release the body.
The poll_frame implementation parks the body until a signal is received/cancelled or a configured timeout elapses.

Tests and a small example are added which demonstrate the behavior.

Closes hyperium/hyper#3833

[seanmonstar]

Why accept an Option? Should we not nudge people harder to supply a timeout?

[bountis]

Hmm, yes, this should probably be stricter and require a timeout as the RFC also states that client SHOULD NOT wait for an indefinite period before sending the content.

I can modify wrap then to accept a timer + a duration. ExpectContinueBody::new can probably continue to accept the option since it's private to the crate and the signal/cancel tests use None, or do you want me to change that too?