Reduce Example: Minimal HTTPS client

[reneme]

Description

This reduces the example introduced in #3901 by introducing some convenient defaults in the ASIO stream. I think it is making a relevant point, that one can write a basic HTTPS client using Botan's TLS in just a few lines of C++20 that works out of the box.

In detail, I added:

• Default_Credentials_Manager that uses System_Certificate_Store as the default trust store (if available)
  The existing example had to provide this overload (System_Credentials_Manager). If no interface to the system's certificate store is implemented on the target system, the client won't be able to establish trust. We need to document this well enough.
• a convenience constructor for TLS::Stream<> that uses defaults for all aspects of the stream's Context object, namely:
  • AutoSeeded_RNG
  • Default_Credentials_Manager
  • TLS::Session_Manager_In_Memory and
  • TLS::Default_Policy

[coveralls]

coverage: 92.004% (-0.02%) from 92.028%
when pulling 63de52d on Rohde-Schwarz:example/minimal_https_client
into bf86f1a on randombit:master.

[reneme]

Mhh, the boost version on CI doesn't seem to be recent enough for this example. I'll look into that later.

[randombit]

This Default_Credentials_Manager seems odd to me tbh. If the system certs are not available, it seems like nothing will work anyway. Why not just require the system certs for the example?

[reneme]

Currently, the Credentials_Manager base class is mostly an interface without useful default implementations. Therefore, users are forced to provide their own Credentials_Manager subclass, even if they just want to use system certificates, anyway.

In my opinion that adds quite a bit of friction. Especially, for people that want to try the ASIO stream as replacement for the OpenSSL stream shipping with Boost. Hence, this is an attempt to have a functional default for the TLS::Context object with minimal parameters.

Admittedly, the Default_Credentials_Manager was just the shortest path to get this. Perhaps, it could be more generic and (optionally) accept user-provided roots. If no root is provided, it could default to the system certs (when available). Like so:

/**
 * @param server_info  the hostname you're trying to connect to
 * @param root_certs   the root certificates you're willing to trust
 *                     (defaults to the operating system's certificates)
 */
Context(Server_Information server_info = Server_Information(),
        std::vector<X509_Certificate> root_certs = {}) :
    m_credentials_manager(std::make_shared<Default_Credentials_Manager>(std::move(root_certs))),
    [...]

[reneme]

Note: I rebased this onto #3901 which adds the very same example but without the convenience additions in this PR.

[reneme]

I moved the Default_Credentials_Manager out of the public interface and demoted it to an implementation detail. See here: #3799 (comment).

[reneme]

Let me dig this up again. The actual example that was initially part of this PR is already merged in #3901.

This now just reduces this example by introducing a convenience constructor for the TLS::Context class used by the ASIO stream. It allows users to get started with a simplistic HTTPS client application with minimal Botan-related boilerplate. (The gist of it now fits on a slide, for instance. Which would come in handy for our contribution to the "BSI IT-Sicherheitskongress" in May 😅.)

Before, the Default_Credentials_Manager was in a public header. But given its slim utility, I degraded it to an implementation detail of said convenience constructor. In the future we could extend this with a similar convenience constructor for a server use case (i.e. just take a cert and a public key).