E2E test: Implement group comparison

Description of the test

Add group label migration validation.

• Refactor label classes, introduce BaseLabel class to share logic
• Move GroupLabel class out of EE as it is all tier feature
• Add custom == method to BaseLabel to be able to compare labels directly based on api response

Check-list

• Confirm the test has a testcase: tag linking to an existing test case in the test case project.
• Note if the test is intended to run in specific scenarios. If a scenario is new, add a link to the MR that adds the new scenario.
• Follow the end-to-end tests style guide and best practices.
• Use the appropriate RSpec metadata tag(s).
• Ensure that a created resource is removed after test execution. A Group resource can be shared between multiple tests. Do not remove it unless it has a unique path. Note that we have a cleanup job that periodically removes groups under gitlab-qa-sandbox-group.
• Ensure that no transient bugs are hidden accidentally due to the usage of waits and reloads.
• If the test requires an admin's personal access token, ensure that the test passes on your local with and without the GITLAB_QA_ADMIN_ACCESS_TOKEN provided.