Add DynamicEntityReferenceItemNormalizer for serialization module

#2827164: Entity reference field normalization has target_type and target_uuid, but not used in denormalization finally landed! This issue can now continue :)

#2970836: Argument 3 passed to Drupal\hal\Normalizer\EntityReferenceItemNormalizer::__construct() must implement interface Drupal\Core\Entity\EntityTypeManagerInterface or be null happened, and made the normalizer specifically conditionally only load for hal. I guess this is a low priority. Understandably so of course!

I'm running into issues with a project that's using Tome for static site generation. When calling drush tome:install everything works fine importing the content until it tries to generate serialized entities that are using dynamic entity reference. The error that gets thrown is

he command "/usr/local/bin/drush tome:import --yes --uri=default --root=/Users/aaron/sites/tome.test/web" failed.  
                                                                                                                      
  Exit Code: 1(General error)                                                                                         
                                                                                                                      
  Working directory:                                                                                                  
                                                                                                                      
  Output:                                                                                                             
  ================                                                                                                    
                                                                                                                      
   [ERROR] Errors encountered when importing content:                                                                 
                                                                                                                      
   [ERROR] Error when running "/usr/local/bin/drush tome:import-content                                               
           'collection_item.94b3123d-2019-4f6a-8a39-c758d8f3200f'":                                                   
                                                                                                                      
           In EntityReferenceFieldItemNormalizer.php line 83:                                                         
                                                                                                                      
                                                                                                                      
             The field "item" property "target_type" must be set to "" or omitted.     

I'll probably create an issue on the Tome project as well, but I think this might be related to the missing serialization normalizer for DER. Does that sound right? Any chance I could help pitch in to create the patch on this issue if so?

Yes, this issue would need to be resolved. Please close the tome issue.