Expose a `removable` attribute for BillableMembers

When returning BillableMember entities, the frontend will need
to know if each one is removable or not

doc/api/members.md

@@ -289,7 +289,8 @@ Example response:
     "avatar_url": "https://www.gravatar.com/avatar/c2525a7f58ae3776070e44c106c48e15?s=80&d=identicon",
     "web_url": "http://192.168.1.8:3000/root",
     "last_activity_on": "2021-01-27",
-    "membership_type": "group_member"
+    "membership_type": "group_member",
+    "removable": true
   },
   {
     "id": 2,
@@ -300,7 +301,8 @@ Example response:
     "web_url": "http://192.168.1.8:3000/root",
     "email": "john@example.com",
     "last_activity_on": "2021-01-25",
-    "membership_type": "group_member"
+    "membership_type": "group_member",
+    "removable": true
   },
   {
     "id": 3,
@@ -310,7 +312,8 @@ Example response:
     "avatar_url": "https://www.gravatar.com/avatar/c2525a7f58ae3776070e44c106c48e15?s=80&d=identicon",
     "web_url": "http://192.168.1.8:3000/root",
     "last_activity_on": "2021-01-20",
-    "membership_type": "group_invite"
+    "membership_type": "group_invite",
+    "removable": false
   }
 ]
 ```

ee/changelogs/unreleased/vij-add-removable-to-billable-member.yml

+---
+title: Expose whether a BillableMember is removable
+merge_request: 61566
+author:
+type: changed

ee/lib/ee/api/entities/billable_member.rb

@@ -7,6 +7,7 @@ module EE
         expose :public_email, as: :email
         expose :last_activity_on
         expose :membership_type
+        expose :removable
 
         private
 
@@ -17,6 +18,10 @@ module EE
           return 'project_invite' if user_in_array?(:shared_project_user_ids)
         end
 
+        def removable
+          user_in_array?(:group_member_user_ids) || user_in_array?(:project_member_user_ids)
+        end
+
         def user_in_array?(name)
           options.fetch(name, []).include?(object.id)
         end

ee/spec/lib/ee/api/entities/billable_member_spec.rb

@@ -47,18 +47,22 @@ RSpec.describe ::EE::API::Entities::BillableMember do
   context 'with different group membership types' do
     using RSpec::Parameterized::TableSyntax
 
-    where(:user_ids, :expected_value) do
-      :group_member_user_ids   | 'group_member'
-      :project_member_user_ids | 'project_member'
-      :shared_group_user_ids   | 'group_invite'
-      :shared_project_user_ids | 'project_invite'
+    where(:user_ids, :membership_type, :removable) do
+      :group_member_user_ids   | 'group_member'   | true
+      :project_member_user_ids | 'project_member' | true
+      :shared_group_user_ids   | 'group_invite'   | false
+      :shared_project_user_ids | 'project_invite' | false
     end
 
     with_them do
       let(:options) { super().merge(user_ids => [member.id]) }
 
-      it 'returns the expected value' do
-        expect(entity_representation[:membership_type]).to eq expected_value
+      it 'returns the expected membership_type value' do
+        expect(entity_representation[:membership_type]).to eq membership_type
+      end
+
+      it 'returns the expected removable value' do
+        expect(entity_representation[:removable]).to eq removable
       end
     end