Adds new and edit modules FF store

What does this MR do?

This is part of https://gitlab.com/gitlab-org/gitlab-ee/issues/8621 & https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9228

This merge request adds 2 modules to the Feature Flags store:

• new
  • it allows a new feature flag to be created - POST
• edit
  • it allows fetching the existing data to prefill the form - GET
  • it allows updating the feature flag - PUT

Although the state between them is similar there are a few differences, it seemed clear to have two different modules than a lot of if/case conditions.

What are the relevant issue numbers?

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

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides