[Process] Consider allowing streamable input

[schmittjoh]

Currently, the process component assumes that input is known before a command is invoked. Sometimes, you want to stream the input, i.e. gradually write it to the process that is being run. This could for example be done by exposing the pipes, or maybe better a specific method for writing:

$process = new Process('takes-some-input');
$process->start();

while ($stream->hasContent()) {
   $process->write($stream->read());
}

[sstok]

I like this idea 👍 and I already have a good use-case: Git rebase interactive.

[javiereguiluz]

Closing it as fixed by #18386.