[Forwardport] Extend default config instead overwrite

[chirag-wagento]

Original Pull Request

#16001

Description

Default FormElementDependenceController config should be extended by custom config but not overridden.

Manual testing scenarios

1. Add custom options to FormElementDependenceController config;
2. Open System config page.

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)

[VladimirZaets]

Hi, @chirag-wagento, I took your PR into processing, thank you for collaboration.

[magento-engcom-team]

Hi @chirag-wagento. Thank you for your contribution.
Changes from your Pull Request will be available with the upcoming 2.3.0 release.