[php-slim4] Add dependency injection container 2

Created by: ybelenko

Adds PHP-DI package and basic app config. It's my second PR with huge refactoring, I've decided to not create generator option for that feature since I don't want to support two different builds. Also I removed TokenAuthentication and Mocker for now to make review easier. I will add these 2 middlewares back in the next PR.

I need to mention two repos which I used as references:

• slimphp/Slim-Skeleton
• odan/slim4-skeleton

So thanks to @odan and @l0gicgate for a good skeletons.

Closes #6852 (closed)

Upgrade note will be added soon.

cc @jebentier @dkarlovi @mandrean @jfastnacht @ackintosh @renepardon

PR checklist

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  ./bin/generate-samples.sh
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