Add PHP 8 Union Types Support for function Parameters

[karyna-t]

In PHP 8, new Union types feature was introduced.
(see details https://wiki.php.net/rfc/union_types_v2)

e.g. it's possible to create a function with a signature like
function baz(Foo|Bar|int $arg) { ...

Right now Magento will not work properly with such a feature. There are several places where we're using Parameter's Class (e.g. for code generation):

• https://github.com/magento/magento2/blob/platform-health/lib/internal/Magento/Framework/Code/Generator/EntityAbstract.php#L338
• https://github.com/magento/magento2/blob/platform-health/lib/internal/Magento/Framework/Code/Reader/ArgumentsReader.php#L105
• https://github.com/magento/magento2/blob/platform-health/lib/internal/Magento/Framework/Code/Reader/ClassReader.php#L36
• https://github.com/magento/magento2/blob/platform-health/lib/internal/Magento/Framework/Code/Reader/SourceArgumentsReader.php#L69
• https://github.com/magento/magento2/blob/platform-health/lib/internal/Magento/Framework/Interception/Code/InterfaceValidator.php#L175
• https://github.com/magento/magento2/blob/platform-health/lib/internal/Magento/Framework/TestFramework/Unit/Helper/ObjectManager.php
  (better to re-check other places)

These places should be adjusted to work properly with union types.
The main reason for this adjustments it's because 3rd-party developers may develop some new extensions and declare them to support only php >= 8.0.
in such modules they may use this PHP8-feature.

Expected result (*)

Magento should not break when union types are used in the function parameters. It should properly generate code and so on.

Actual result (*)

Right now Magento can't handle union types introduced in php 8.

Related issues:

#34264
#34265

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