Add a compat trait to address B/C issues with EntityRepository

[mbabker]

Ref: #2708

In ORM 3.0, the EntityRepository::__call() method signature is fully typehinted. As it includes a mixed return type, that signature can't just be applied to the package right now due to PHP 7.4 still being supported.

To help with B/C, an internal trait is introduced that any entity repository provided by this package can use (thus far, it only affects the NestedTreeRepository, but if we find other gaps, the trait can be updated to account for those as well.

[codecov]

Codecov Report

Attention: 12 lines in your changes are missing coverage. Please review.

Comparison is base (66e6a60) 79.33% compared to head (01b535b) 79.26%.

Files	Patch %	Lines
...rc/Tree/Entity/Repository/NestedTreeRepository.php	73.17%	11 Missing ⚠️
src/Tool/ORM/Repository/EntityRepositoryCompat.php	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2758      +/-   ##
==========================================
- Coverage   79.33%   79.26%   -0.07%     
==========================================
  Files         162      163       +1     
  Lines        8506     8517      +11     
==========================================
+ Hits         6748     6751       +3     
- Misses       1758     1766       +8     

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[franmomu]

Thanks @mbabker! I guess we should use this strategy with more classes to deal with BC with ORM 3.