How to extend Save.php action of Creditmemo using plugin for CreditmemoItem in magento 2?

di.xml

< ?xml version="1.0"?>

    
        
        
    

extension_attributes.xml

< ?xml version="1.0"?>

    
        
    

Save.php

< ?php

namespace CCreditMemoItemDetailsModelPlugin;

use MagentoFrameworkExceptionCouldNotSaveException;

class Save
{
    public function beforeSave(
        MagentoSalesApiCreditMemoItemRepositoryInterface $subject,
        MagentoSalesApiDataCreditMemoItemInterface $entity
   )
   {
     echo("hello");exit();
   }


    public function afterSave(
        MagentoSalesApiCreditMemoItemRepositoryInterface $subject,
        MagentoSalesApiDataCreditMemoItemInterface $result
    ) {            
       echo("hello");exit();

        $result = $this->savecreditmemoitemdetailsAttribute($result);

        return $result;
    }

    private function savecreditmemoitemdetailsAttribute(MagentoSalesApiDataCreditMemoItemInterface $creditmemoitemdetails)
    {
        $extensionAttributes = $creditmemoitemdetails->getExtensionAttributes();
        if (
            null !== $extensionAttributes &&
            null !== $extensionAttributes->getcreditememoitemdetailsAttribute()
        ) {
            $request = $this->getRequest();
            $data = $request->getPost('creditmemo');
            $data['creditmemo']['refund_creditlof_refund_credit'] = 1;
            $data['do_offline'] = true;
            $request->setPostValue('creditmemo', $data);
            $creditememoitemdetailsAttributeValue = $extensionAttributes->getcreditememoitemdetailsAttribute()->getValue();
            try {
                // The actual implementation of the repository is omitted
                // but it is where you would save to the database (or any other persistent storage)
                //$this->CreditMemoItemDetailsRepository->save($creditmemoitemdetails->getEntityId(), $creditememoitemdetailsAttributeValue);
                foreach($data['item_Number'] as $selected){
                        echo $selected;
                        $this->creditmemoitemdetails->setItemNumber($selected);
                }exit();
                $creditememoitemdetailsAttributeValue->save();
            } catch (Exception $e) {
                throw new CouldNotSaveException(
                    __('Could not add attribute to order: "%1"', $e->getMessage()),
                    $e
                );
            }
        }
        return $creditmemoitemdetails;
    }
}

Basically I am trying to extend Save.php by creating this plugin and save custom data in my newly created table in backend. But when I am saving creditmemo it is not passing any of those exists nor it gives any warnings or error. What should I do at this point? Am I doing it in right manner I am referring to How to extend creditmemo save action in magento 2 and https://store.fooman.co.nz/blog/an-introduction-to-extension-attributes.html