[Backport] Added form fieldset before html data to \Magento\Framework\Data\Form\Element\Fieldset in getElementHtml() method

[vasilii-b]

Original Pull Request

#18798

Description

In \Magento\Framework\Data\Form\Element\Fieldset class, there is not called the getBeforeElementHtml() method in getElementHtml(). This stops developers from adding needed information before the fieldset in a easy way.

Fixed Issues (if relevant)

1. Class \Magento\Framework\Data\Form\Element\Fieldset breaks specification of the parent class \Magento\Framework\Data\Form\Element\AbstractElement by not calling the method getBeforeElementHtml (getAfterElementHtml is called) #2618

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 @vasilii-b. Thank you for your contribution
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[magento-engcom-team]

Hi @VladimirZaets, thank you for the review.
ENGCOM-3359 has been created to process this Pull Request

[magento-engcom-team]

Hi @vasilii-b. 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.