Make ReadWritePropertiesExtensionProvider part of API

[olsavmic]

Custom rules as such https://github.com/shipmonk-rnd/phpstan-rules/blob/master/src/Rule/UselessPrivatePropertyNullabilityRule.php need access to registered ReadWritePropertiesExtension[] for seamless configuration.

[ondrejmirtes]

I did something that makes more sense to me: 49c27d8

You no longer have to pass $extensions to ClassPropertiesNode::getUninitializedProperties(). You can omit this argument and everything will work as expected :)

If you have extensions referenced in a rule test case, move them from the rule constructor to a new overriden method getReadWritePropertiesExtensions.