Support CURD operation for feature flag scopes (GitLab API)

What does this MR do?

Previously, in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9110/pipelines, we supported multiple environment feature for Unleash API. So that unleash clients can use multiple environment feature today, although, users cannot add/remove environment scopes in GitLab UI yet.

This merge request exposes attributes for scope creation in order to let users create/update/destroy scopes.

What are the relevant issue numbers?

Related: https://gitlab.com/gitlab-org/gitlab-ee/issues/8621

This MR is built on top of https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9110

This feature is behind a feature flag (Flipper) named feature_flags_environment_scope

To enable this feature, execute FeatureFlag.enable(:feature_flags_environment_scope).

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [-] Documentation created/updated via this MR
• Tests added for this feature/bug
• [-] Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• [-] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• [-] For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer