[Backport] Adding trimming sku value function to sku backend model.

[gelanivishal]

Original PR

#18019

Description

I have added function to sku backend model which trimms value from whitespace.

Fixed Issues (if relevant)

1. Trim whitespace on SKU when saving product #16572 Trim whitespace on SKU when saving product
2. SKU values are not trimmed with the space. #12300 SKU values are not trimmed with the space.

Manual testing scenarios

1. Edit sku, by adding some whitespace
2. Save product.
3. Sku will be trimmed and saved in changed form.
4. Messsage that sku has been changed will be displayed.

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)

[magento-engcom-team]

Hi @gelanivishal. Thank you for your contribution
Here is some useful tips how you can test your changes using Magento test environment.
Add the comment under your pull request to deploy test or vanilla Magento instance:

• @magento-engcom-team give me test instance - deploy test instance based on PR changes
• @magento-engcom-team give me $VERSION instance - deploy vanilla Magento instance

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[magento-engcom-team]

Hi @VladimirZaets, thank you for the review.
ENGCOM-3317 has been created to process this Pull Request

[magento-engcom-team]

Hi @gelanivishal. Thank you for your contribution.
We will aim to release these changes as part of 2.2.8.
Please check the release notes for final confirmation.

Please, consider to port this solution to 2.3 release line.
You may use Porting tool to port commits automatically.