Add httpsoft/http-message to php-http/discovery

[BoShurik]

See https://docs.php-http.org/en/latest/discovery.html

[devanych]

@BoShurik hi! You mean:

"provide": {
    "php-http/message-factory-implementation": "1.0"
},

well, and all related of course?

[BoShurik]

I mean php-http/discovery#241 :)