Customize PHP memory limit for Horizon

[rpungello]

With Horizon right now, you can specify a memory cap, but this only seems to come into play after each job where the supervisor will restart the individual processes if they've exceeded the specified cap.

So far as I can tell, there is no way to actually alter the memory PHP is allowed to use at runtime. While you can use php.ini to set memory_limit=X, this gets a little tedious for apps with a single consolidated Docker image. PHP has the ability to do this by doing, for example, php -d memory_limit=X artisan horizon. The problem is this only affects the supervisor process, not the individual workers.

Thoughts on having a new setting in the Horizon options that allow us to customize this value at runtime? Maybe 'phpMemory' => '1G', for example. This would then be used when launching the individual worker processes by calling php -d memory_limit=1G artisan horizon:work[...]

Happy to work on a PR if this makes sense.