[Backport] Fix issue with unexpected changing of subscription status after customer saving

[arendarenko]

Original Pull Request

#18488

Description

I found that every time when customer data is saving, Newsletter module triggers "afterSave" (https://github.com/magento/magento2/blob/2.3-develop/app/code/Magento/Newsletter/Model/Plugin/CustomerPlugin.php#L67) plugin for CustomerRepository, which updates customer subscription status. According to this plugin code, it has two customers in arguments: customer that was actually passed to save (it called $customer) and customer which was modified after during save method (it called $result). Those two variables are comparing their "extensionAttributes" which are setting in the same plugin by method "afterGetById" (https://github.com/magento/magento2/blob/2.3-develop/app/code/Magento/Newsletter/Model/Plugin/CustomerPlugin.php#L151). And if they are exact - module didn't changes customer subscription, in other case - it did.

The problem is that "save" method triggers afterGetById only for $customer, but not for $result (which is actually users "get" method: https://github.com/magento/magento2/blob/2.3-develop/app/code/Magento/Customer/Model/ResourceModel/CustomerRepository.php#L253). In this case, extensionAttributes are always different for $customer and $result, and this is why every time when "afterSave" plugin triggers, module tries to make subscribe/unsubscribe user depends on it current subscription status.

Manual testing scenarios

1. Create new customer
2. Go to backend -> Customers -> Edit -> and open previously created customer for edit
3. Create breakpoint via XDebug on https://github.com/magento/magento2/blob/2.3-develop/app/code/Magento/Newsletter/Model/Plugin/CustomerPlugin.php#L83
4. Edit customer personal data (only) and click Save
5. Compare property data of variables $newExtensionAttributes and $initialExtensionAttributes.

Expected result:
Both variables would have the same extension attributes.

Actual result:
$newExtensionAttributes have extension attributes, but $initialExtensionAttributes is not (empty array).

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

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