[PHP Provider] Ensure that PHP-FPM workers output is streamed to Nginx

[lolautruche]

This PR allows errors and warnings from PHP-FPM workers to be streamed to Nginx output. This is critical to be able to get access to the application log, which is usually streamed on php://stderr in production.

Reference: https://serverfault.com/questions/417102/nginx-not-logging-php-errors

[coffee-cup]

The PR looks good. Can you please run cargo snapshot in the root to update the tests

[lolautruche]

@coffee-cup Sorry, I'm a bit ignorant in Rust 😇 .
I did run cargo snapshot, but it didn't update anything:

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 1 filtered out; finished in 0.00s

done: no snapshots to review

I'm sure I'm missing something 😅

[lolautruche]

Nevermind, I figured it out :-)