feat(http1): support expect: 100-continue on the client

[bountis]

Handle the Expect: 100-continue header fully.
https://www.rfc-editor.org/rfc/rfc9110.html#name-expect

This adds the missing handling logic of the header. The client didn't handle the header previously.

If there's the Expect: 100-continue header present the client only writes the head and parks the body.
If the server returns 100 Continue then go on and write the body.
If the server returns any non-1xx response the client drops the body and closes the connection.
Empty body ignores the header completely.

Also add a configurable timer to the client, client sends the body after the timeout, only if there's a timer setup, if not then it waits indefinitely.

Closes #3833

[bountis]

Had to include this before the parse result match because otherwise the early return in the Pending arm would never flip the state and deadlock.

[seanmonstar]

Thanks for the PR! One thing I'll note, what I hoped to describe in #3833 is that this can be done completely outside of hyper, and we could provide helpers to do so in hyper-util. Would you be up for trying to port to helpers over there?

[bountis]

Thanks for the PR! One thing I'll note, what I hoped to describe in #3833 is that this can be done completely outside of hyper, and we could provide helpers to do so in hyper-util. Would you be up for trying to port to helpers over there?

Ah right, jumped into implementing without reading thoroughly enough. Ye no problem, will do.