Add `all` value support

ee/app/graphql/types/epic_state_enum.rb

@@ -5,6 +5,7 @@ module Types
     graphql_name 'EpicState'
     description 'State of a GitLab epic'
 
+    value 'all'
     value 'opened'
     value 'closed'
   end

ee/spec/graphql/types/epic_state_enum_spec.rb

@@ -6,6 +6,6 @@ describe GitlabSchema.types['EpicState'] do
   it { expect(described_class.graphql_name).to eq('EpicState') }
 
   it 'exposes all the existing epic states' do
-    expect(described_class.values.keys).to include(*%w[opened closed])
+    expect(described_class.values.keys).to include(*%w[all opened closed])
   end
 end