Allow the mapped event subscriber to use an attribute reader by default

[mbabker]

This is building off the conversation in #2683 as a way to try and decouple the hard doctrine/annotations dependency in Gedmo\Mapping\MappedEventSubscriber. Whenever any of the extension listener classes don't have a reader injected through the setAnnotationReader() method, this base class will call Gedmo\Mapping\MappedEventSubscriber::getDefaultAnnotationReader() and create a basic annotation reader. With the announced intent to deprecate the doctrine/annotations package, this is a step in helping to break the hard dependency here by introducing a new fallback behavior if doctrine/annotations isn't installed (right now it's a hard dependency to this package so these new fallbacks won't be hit at all yet, hence this being a WIP PR with time to discuss it).

For B/C, it will continue to default to creating an annotation reader first. It then falls back to creating an attribute reader if running PHP 8, then will hard fail if neither option is available.

A reader still needs to always be created for now as the $annotationReader argument for the Gedmo\Mapping\ExtensionMetadataFactory class constructor requires a reader, the logic in the subscriber's getDefaultAnnotationReader() method can be revisited when this hard requirement is addressed (as the factory only needs a reader if it creates a Gedmo\Mapping\Driver\AnnotationDriverInterface or Gedmo\Mapping\Driver\AttributeDriverInterface instance; XML or (deprecated) YAML drivers do not need this reader).

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (9d5c15d) 79.33% compared to head (8a1f28b) 79.24%.

Files	Patch %	Lines
src/Mapping/MappedEventSubscriber.php	50.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2713      +/-   ##
==========================================
- Coverage   79.33%   79.24%   -0.10%     
==========================================
  Files         162      162              
  Lines        8460     8471      +11     
==========================================
+ Hits         6712     6713       +1     
- Misses       1748     1758      +10     

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[franmomu]

thanks @mbabker!