Allow install profiles to change the system requirements

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FYI didn't read all the comments but #56 appeared to be a rebase.

This could use an issue summary update as I'm not fully understanding the use case or the "why" maybe the test case will have show that. But the issue summary should mention the proposed solution and other fields per default template.

Updated the MR
- Fix for php_memory_limit
- Test for overriding PHP version and memory limit

Update also the issue summary

Cleaning up tags.

Think the last thing is to get a change record written with an example how this would be set.

Unfortunately, i think the MR is not ready ... yet

2 things:
1. It would be nice to have also RECOMMENDED_PHP ... overridden.
2. What if the values are less than "default" - Drupal provided? Should we allow that.

Another thought:
Wondering if Drupal\Core\Utility\PhpRequirements shouldn't be the place or getting the "right" PHP Requirements
instead of system.install ...

but maybe i'm just "overthinking" ...