Documentation: Examples of HTTPS clients using ASIO

[reneme]

This adds another example that uses C++20 coroutines to implement a basic HTTPS client. Note that this example needs a fairly recent version of Boost: 1.81.0 (December 2022). Use it like so: ./tls_stream_coroutine_client botan.randombit.net 443 /news.html. It also adapts the existing example (tls_stream_client), that also uses ASIO but with callback handler methods instead of coroutines. That may now also be called with a user-defined host and HTTP target.

Note that the coroutine example is pretty much the same as in #3799 but with less "convenience additions" to the library. Namely, the deduction guides for TLS::Stream, only.

[coveralls]

coverage: 92.012% (+0.02%) from 91.996%
when pulling 69235b2 on Rohde-Schwarz:doc/https_clients
into 7f15ce7 on randombit:master.

[FAlbertDev]

I like the new client example 👍

[reneme]

@randombit Is that okay to merge, or would you prefer to keep the master branch pristine in case we need to follow up with a 3.3.1 release? We'd love to have this upstream, because it is part of a contribution the the "BSI Kongress" (in German). The paper deadline is on Monday, and I'd like to place a perma-link to the example. :)