Marked Translation class as implementing Stringable

[Steveb-p]

Question	Answer
JIRA issue	N/A
Type	improvement
Target Ibexa version	v3.3
BC breaks	no

This PR adds Stringable as phpdoc annotation to eZ\Publish\API\Repository\Values\Translation.

Reasoning

When working with content validation it become apparent for me that this class is lacking Stringable interface. Ibexa\Core\FieldType\ValidationError::getTranslatableMessage returns a Translation object that - judging by our implementations: Plural & Message - should always have __toString() method.

Of course, Stringable did not exist at the moment when this code was written.

However, because there is no guarantee that user code that extends it does indeed implement this method, and because Stringable interface is available from PHP 8.0, I'm opting to mark the class as implementing it via phpdoc instead. Otherwise we could break user code or require a polyfill (which is in itself an option, due to it's lightweight nature).

We cannot simply add this method to Translation without hiding the fact that implementation is missing, because as per php docs:

Warning
It was not possible to throw an exception from within a __toString() method prior to PHP 7.4.0. Doing so will result in a fatal error.

I propose that for 3.3:

• We add a phpdoc to Translation denoting the intent of implementing Stringable

but for 4.x:

• We add a polyfill requirement for symfony/polyfill-php80
• Directly implement Stringable in Translation
• Add an exception throwing implementation __toString() method, so that user code does not break.

Checklist:

• Provided PR description.
• Tested the solution manually.
• Provided automated test coverage.
• Checked that target branch is set correctly (master for features, the oldest supported for bugs).
• Ran PHP CS Fixer for new PHP code (use $ composer fix-cs).
• Asked for a review (ping @ezsystems/engineering-team).

[sonarqubecloud]

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[alongosz]

@Steveb-p I'm all for it if it doesn't break static analysis. Or is the issue outdated since it was forgotten? :D