Design document for the new HTTP API (#2971)

* Add design document for the new HTTP API

* Add design details about Sockets, Client and other APIs

Also, move examples to their specific section.

* Slightly reword Go net/http section

* Add socket done blocking call to TCP example

* Update IPC example to match the network API

* Apply suggestions from Mihail

Co-authored-by: Mihail Stoykov <312246+mstoykov@users.noreply.github.com>

* Add introduction to Design section

* Reference Streams API issue

* Remove mention of replacing k6/http

* Suggest merging into k6 core as experimental earlier, remove phase 6

Resolves #2971 (comment)

* Add mockup for an Event system

This could be a way to address features like 2667 and 1716.

* Replace dial* functions with <protocol>.open()

Resolves #2971 (comment)

* Remove sleep from example

* Add note about the WIP state of the proposal

* Rework phase 1 section, prioritize #761

Addresses #2971 (comment)

* Add TLS and HTTP/2 examples, remove HTTP/3 example

This changes the way the HTTP client obtains the socket, following this
discussion[1]. Instead of passing one socket, a dial function can be set
to control how the client creates the socket.

The HTTP/3 example was removed since it's too early to determine that API.

[1]: #2971 (comment)

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Co-authored-by: Mihail Stoykov <312246+mstoykov@users.noreply.github.com>