[php-slim4] Move config to a separate file

[ybelenko]

I've used current Slim4 server in my personal project and I think that config file should be extracted from index.php. The most suitable format is config.inc.php.

I've added config folder with example.inc.php. User should copy that file to config/dev/config.inc.php and config/prod/config.inc.php. I prefer config.inc.php format over .env and .ini because these formats can be used only for flags or string variables. So config.inc.php file is the only way if you need to specify callback:

return [
    'getPDOConnection' => function () {
        return new PDO('mysql:dbname=dbname;host=localhost;charset=UTF8', 'root', 'root');
    },
];

Closes #6854

PR checklist

• Read the contribution guidelines.
• If contributing template-only or documentation-only changes which will change sample output, build the project beforehand.
• Run the shell script ./bin/generate-samples.shto update all Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master. These must match the expectations made by your contribution. You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/config/java*. For Windows users, please run the script in Git BASH.
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cc @jebentier @dkarlovi @mandrean @jfastnacht @ackintosh @renepardon

[ybelenko]

cc Slim4 team member @l0gicgate

[ybelenko]

Notice to reviewers! Don't mind force push please, I did git rebase master to fix merge conflict and keep clean git commit history. There are no changes in PR itself.

[ybelenko]

I guess silence means that all reviewers agree to this changes 😄

@wing328 could you merge it when you have time? I have two coming up PRs waiting.