33524 custom autoload

1. Why is this change necessary?

The google sdk brings his custom GuzzleHttp library in version ^6.0 but Shopware uses 5.3^. This results into a compatibility issue with BestitKlarnaPayments

2. What does this change do, exactly?

I have created a custom autoloader which will load all requires libraries, except the custom GuzzleHttp one.

3. Describe each step to reproduce the issue or behaviour.

• Open your browser and navigate to https://github.com/shopware/shopware/tree/5.4
  • Download ZIP
  • Extract ZIP
  • Copy "tests"-Directory into your shopware installation
• Install "phpunit" if not already
  • Use version ^5.0 for Shopware 5.4 and below
  • Use version >=6.0 for Shopware 5.5 and above
• Run "phpunit" inside the plugin directory
  • Due autoloading tests you need to test each file in Tests/Functional/*Test.php, otherwise the test will fail
• Besides the custom autoloader, the general plugin functionality (within NetiSocialLogin) needs to be tested.

4. Please link to the relevant issues (if any).

https://redmine.netinventors.de/issues/33524

5. Checklist

• I have updated the plugin version and changelog
• I have set the required labels
• I can confirm the code style fits our requirements
• I can confirm no debug output is generated anywhere