Add VAPID public key to instance serializer (#28006)

Co-authored-by: Renaud Chaput <renchap@gmail.com>

spec/serializers/rest/instance_serializer_spec.rb

@@ -10,5 +10,11 @@ describe REST::InstanceSerializer do
     it 'returns recent usage data' do
       expect(serialization['usage']).to eq({ 'users' => { 'active_month' => 0 } })
     end
+
+    it 'returns the VAPID public key' do
+      expect(serialization['configuration']['vapid']).to eq({
+        'public_key' => Rails.configuration.x.vapid_public_key,
+      })
+    end
   end
 end