[REQ][php] declareStrictTypes option for all PHP generators

Is your feature request related to a problem? Please describe.

Usually I totally against codegen options which modifies one line of code, but this one looks really handy.

Describe the solution you'd like

Add declareStrictTypes switch option to AbstractPhpCodegen class, then:

openapi-generator-cli generate -g php --additional-properties=declareStrictTypes=true \
    -o out -i petstore.yaml

then in all mustache files:

<?php 

{{#declareStrictTypes}}declare(strict_types=1);{{/declareStrictTypes}}

We also need to decide whether this switch option should be on or off by default. I think it should be on, but it can cause breaking changes.

Describe alternatives you've considered

Close this issue as not necessary feature 😆

Additional context

cc @jebentier @dkarlovi @mandrean @jfastnacht @renepardon