[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

Start with the AbstractPhpCodegen class and then inspect the PHP mustache files used by the generators. Trace how additional properties are exposed to templates, determine the default for declareStrictTypes, and verify that the option consistently controls the strict-types declaration across the relevant generated PHP files.