post-processing: run formatter after client generation

[szepeviktor]

Generator output:

[main] INFO  o.o.c.languages.AbstractPhpCodegen - Environment variable PHP_POST_PROCESS_FILE not defined so the PHP code may not be properly formatted. To define it, try 'export PHP_POST_PROCESS_FILE="/usr/local/bin/prettier --write"' (Linux/Mac)
[main] INFO  o.o.c.languages.AbstractPhpCodegen - NOTE: To enable file post-processing, 'enablePostProcessFile' must be set to `true` (--enable-post-process-file for CLI).

[szepeviktor]

Yet another openapi issue blocking this.
php-cs-fixer is not able to fix that docblock.