fix phpdoc for ModelToIdPropertyTransformer::$property + add test

[dmaicher]

Subject

I am targeting this branch, because its BC.

Related to #6779 (fix needs to be merged into master first).

Changelog

### Fixed
- fixed phpdoc for `Sonata\AdminBundle\Form\DataTransformer\ModelToIdPropertyTransformer::$property` which can be `string[]` as well

[VincentLanglet]

@phpstan-template P of string|string[] is not supported, so maybe @phpstan-template P ?

[VincentLanglet]

Rebasing 3.x should fix the build

[OskarStark]

Thank you David!

[franmomu]

Just an idea, since ModelToIdPropertyTransformer apparently is only used in ModelAutocompleteType (and the user usually does not interact with ModelToIdPropertyTransformer), what about to only allow array in ModelToIdPropertyTransformer and apply a normalizer to ModelAutocompleteType option (to OptionsResolver::setNormalizer) in case is just a string, change it to array.

So this way, the user can use string|string[] for property and the ModelAutocompleteType is the one in charge of normalize it to string[].