Review HttpClient's ContentProvider

[sbordet]

Jetty version
10

Description
The initial mistake was to model ContentProvider to return an Iterator<ByteBuffer> for the content.

This does not play well because Iterator does not support asynchronous content (e.g. when the client provides some content but not all of it, and then there is a pause: Iterator is not really the right abstraction for this "paused" content).

Furthermore, the type of Iterator being ByteBuffer is not quite enough as sometimes the content is provided with the pair (ByteBuffer, Callback) - completing the callback will recycle the buffer or otherwise signal to provide more content.
This is typical when implementing proxies: the content arriving to the proxy comes already with an attached callback object of some kind to implement backpressure, and we need a way to feed it to the proxy client to send it upstream, and be notified only when the send upstream completed.

Unfortunately, ContentProvider is an API class that users may implement to provide their own content. During its evolution, the initial design showed its limits and for Jetty 10 is probably best to review the design (perhaps in some reactive fashion).

[sbordet]

See also #3756 for additional comments.