Support PSR-17

[Taluu]

Q	A
Bug?	no
New Feature?	yes
Version	latest

Actual Behavior

It doesn't support PSR-17 factories

Expected Behavior

It should support PSR-17 factories (such as @Nyholm's psr-7 package, as the httpplug is already supported)

Possible Solutions

Allow to discover the psr-17 interfaces and the implementations that supports those (or at least the most popular ones, such as @Nyholm's psr-17 and zend diactoros - didn't look at the others)

[sagikazarmark]

Thanks, we are working on it.

[dbu]

in php-http/message#105 we say we want to drop our own factories as they are superseeded by PSR-17. i think it makes sense to change this library to discover PSR-17 factories instead of httplug factories for its version 2. (we can't discover both httplug factories and PSR-17 factories because httplug does not extend PSR-17 and therefor can't be mixed)

HttpClientDiscovery should switch to discover a PSR-18 client, but HttpAsyncClientDiscovery would stay on Httplug because there is no PSR for asynchronous clients.