CMS 6: Remove explicit dependency for Guzzle - use php-http/discovery instead.

[GuySartorelli]

Currently some of our modules have guzzle as an explicit dependency.

We should use php-http/discovery instead, which allows developers to use whichever PSR18-compatible library they want.
See https://docs.php-http.org/en/latest/discovery.html#using-discovery-in-a-shared-library which explains how to set this up, and some of the features such as automatically installing a compatible PSR18 library if one is not explicitly declared, and using Psr18Client as the runtime class which internally "will use whatever PSR-7, PSR-17 and PSR-18 implementations your users have installed."

Useful links

• https://docs.php-http.org/en/latest/discovery.html#using-discovery-in-a-shared-library
• https://github.com/php-http/discovery

Acceptance criteria

• The discovery pattern is used so that any library supplying an appropriate HTTP client can be used
• No supported modules have a specific HTTP client in their composer dependencies
• Any existing YAML config or PHP code that expects a specific HTTP client is updated to use and respect the relevant PSR interface.