Add environment scope to CI group variables

What does this MR do?

Related to #2874 (closed)

• Add environment_scope to ci_group_variables table
• Replace unique "group_id & key" index with "group_id & key & environment_scope"
• Hide this feature behind a FF ci_group_variable_environment_scope

TODO:

• We may need changes after new_variables_ui ❗

NOTES

• database review is required
• frontend review is required
• documentation review is required, this is a new feature with feature flag .
• backend : Some tests are just copied from project CI variables. We can unify them if that's better 🤔

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team