[Backport] Fix type hints and add undefined property in Webapi [2.3-develop]

[mageprince]

Original Pull Request

#16135

Description

• Fix type hints in Webapi/Controller/Soap/Request/Handler.php and Webapi/Model/Plugin/GuestAuthorization.php

• Fix incorrect case in property annotation in Soap\Server.php on line 34

• add undefined property _appState in Controller\Soap.php on line 104

• Fix method Magento\Webapi\Model\Soap\Fault::toXml() invoked with 1 parameter, 0 required in Soap\FaultTest.php on line 128

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 @mageprince. 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

For more details, please, review the Magento Contributor Assistant documentation

[magento-engcom-team]

Hi @orlangur, thank you for the review.
ENGCOM-2218 has been created to process this Pull Request

[magento-engcom-team]

Hi @mageprince. Thank you for your contribution.
We will aim to release these changes as part of 2.2.6.
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.