[Translatable] added selective field translation to the translatable extension

[smatyas]

This PR tries to solve the problem where the user reaches the mysql join table count limitation due to many translatable fields.

If we have too many, the following mysql error arises:

ERROR 1116 (HY000): Too many tables; MySQL can only use 61 tables in a join

---

Certainly, it is not a complete solution for the problem, meaning that we will still be unable to translate every field in one query, but I found it really useful in certain scenarios.

Using this optional hint, the user can be selective about the fields to translate, so it will limit the number of joins to those fields.

Without this, the user have to use different workarounds that makes this extension less useful.

---

usage example:

        $query->setHint(Query::HINT_CUSTOM_OUTPUT_WALKER, TranslationWalker::class);
        $query->setHint(TranslatableListener::HINT_FALLBACK, 1);
        $query->setHint(TranslatableListener::HINT_TRANSLATABLE_LOCALE, $locale);

        // Only these fields will be translated by the walker.
        $query->setHint(TranslationWalker::HINT_ONLY_FIELDS, [
            'entity.marketingTitle',
            'entity.marketingDescription',
            'event.name',
            'marketingCategory.name'
        ]);

[codecov]

Codecov Report

❌ Patch coverage is 40.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.82%. Comparing base (0632ab1) to head (05072ed).
⚠️ Report is 134 commits behind head on main.

Files with missing lines	Patch %	Lines
...ranslatable/Query/TreeWalker/TranslationWalker.php	40.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2874      +/-   ##
==========================================
+ Coverage   78.75%   78.82%   +0.07%     
==========================================
  Files         163      167       +4     
  Lines        8593     8643      +50     
==========================================
+ Hits         6767     6813      +46     
- Misses       1826     1830       +4     

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[github-actions]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[smatyas]

We still use a custom walker to mitigate this limitation. Is there a chance to merge this any time? If so, we're willing to cover it with tests.