Add work_item_epics feature flag

What does this MR do and why?

In order to prepare for work item epics GA we need to combine all its functionality under a single feature flag. As a first step, this MR introduces the new flag work_item_epics and updates the checks for displaying namespace-level work items to be available when the new flag is enabled.

This will allow us to access work item epics even if namespace_level_work_items feature flag is disabled.

Also, in this MR we introduce the feature flag create_namespace_level_work_items so we can guard the creation of work items at group level (any type except Epic) separately from namespace_level_work_items and work_item_epics. The creation of WI with type Epic is guarded by the feature flag work_item_epics and a license check for the epics feature.

To summarise:

• The method Group#work_item_epics_enabled? guards displaying work items at the group level, this is enabled when either the namespace_level_work_items OR work_item_epics flag is enabled.
• The ff create_group_level_work_items guards creating group-level work items that are not Epic type
• The work_item_epics also guards creating group-level work items that are type Epic (creation also requires the licensed feature epics to be available)

Follow-ups:

• Update WorkItems::WorkItemsFinder to exclude types that are not enabled: exclude Epic when work_item_epics is disabled and exclude every other type if create_group_level_work_items is disabled. These types should be excluded from the filtering options too.
• Replace other epic work items-related feature flags with the flag work_item_epics

MR acceptance checklist

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✅

How to set up and validate locally

Example below:

1. In rails console enable the new flag work_item_epics and disable namespace_level_work_items and create_group_level_work_items

   Feature.enable(:work_item_epics)
   Feature.disable(:namespace_level_work_items)
   Feature.disable(:create_group_level_work_items)

2. Visit any group's epic and verify that clicking the New epic button opens a pop-up dialogue that creates a work item epic.
3. Verify that the new epic can be viewed with the work item path, e.g http://127.0.0.1:3000/groups/GROUP-PATH/-/work_items/EPIC_IID
4. Visit http://127.0.0.1:3000/-/graphql-explorer and verify that work items can be created only with the type Epic

Click to expand

# Get work item type IDs for EPIC and ISSUE
query getGroupTypes {
  group(fullPath: "flightjs") {
    id
    name
    webUrl
    workItemTypes(name: EPIC) {
      edges {
        node {
          id
          name
        }
      }
    }
  }
}

mutation createEpic {
  workItemCreate(input: {
    namespacePath: "group-a", title: "Epic WI",
    workItemTypeId: "gid://gitlab/WorkItems::Type/8"
  }) {
    errors
    workItem {
      id
      webUrl
      workItemType {
        name
      }
    }
  }
} # Should succeed

mutation createIssue {
  workItemCreate(input: {
    namespacePath: "group-a", title: "Issue WI",
    workItemTypeId: "gid://gitlab/WorkItems::Type/1"
  }) {
    errors
    workItem {
      id
      webUrl
      workItemType {
        name
      }
    }
  }
} # Should return error "create_group_level_work_items feature flag is disabled. Only project paths allowed."

5. Change the feature flags to test the opposite scenario where all types are allowed except Epic.

     Feature.disable(:work_item_epics)
     Feature.enable(:create_group_level_work_items)

6. Use the same queries as step 4 and verify that issue creation succeeds while the epic type returns the "Epic type is not available for the given group" error message.

     Feature.disable(:work_item_epics)
     Feature.enable(:create_group_level_work_items)

Related to #467686