Allow pinning the preferred implementations in composer.json

[nicolas-grekas]

Q	A
Bug fix?	no
New feature?	yes
BC breaks?	no
Deprecations?	no
Related tickets	fixes #223
Documentation	TBD
License	MIT

Add this to your composer.json to pin guzzle as PSR-17 implementation even if nyholm/psr7 is installed:

    "extra": {
        "discovery": {
            "psr/http-factory-implementation": "GuzzleHttp\\Psr7\\HttpFactory"
        }
    }

This also works for single interfaces:

    "extra": {
        "discovery": {
            "Psr\\Http\\Message\\RequestFactoryInterface": "Slim\\Psr7\\Factory\\RequestFactory"
        }
    }

[boesing]

Huge! Thanks.

[dbu]

i am adding some documentation for this in php-http/documentation#305

[dbu]

d'oh, i missed php-http/documentation#304