Create flex recipe

[dbu]

Symfony 4 offers a simplified way to install packages: https://symfony.com/doc/current/setup/flex.html

We should offer a flex recipe for this bundle.

We should install whichever http client symfony suggests by default but rely on the autodiscovery to not have to configure other bundles in the recipe.

If i remember correctly, fabien said he considers adding the recipe for this bundle to the official recipes instead of just contrib.