Allow any psr-18 http client (instead of requiring guzzle)

[tacman]

composer why guzzlehttp/guzzle
league/oauth2-client 2.7.0 requires guzzlehttp/guzzle (^6.0 || ^7.0) 

I prefer to use the Symfony HttpClient, but this package requires guzzle. Any chance it could be more generic, that is, require any PSR-18 client to operate, then the user can install guzzle or whatever they want.

Thanks for your consideration. Not urgent, but it'd be nice to have one less dependency.

[cyraid]

This has been getting me too, as I have to use full out guzzle now since Payum checks if classes exist, and Guzzle is high up there, so it'll of course use that.

(Which also forces me to use php-http/message-factory which is deprecated/abandoned)