[RFC][Process] stdin could be a stream

Hello.

For now, we can not use a `stream` for `stdin`.
See: 
- https://github.com/symfony/symfony/blob/master/src/Symfony/Component/Process/Process.php#L280 (`strlen` is not possible on `stream`)
- https://github.com/symfony/symfony/blob/master/src/Symfony/Component/Process/Process.php#L304 (`substr` is not possible on `stream`)

ping @romainneutron ;)
