[Code Health] Ensure service module msg responses are non-empty (#973)

# Summary

Ensure all supplier module msg responses are non-empty. This adds a
`Service` field to the following protobuf type(s), and updates the unit
tests to assert for presence and correctness.

- `MsgAddServiceResopnse`

## Issue

- #663

## Type of change

Select one or more from the following:

- [ ] New feature, functionality or library
- [ ] Consensus breaking; add the `consensus-breaking` label if so. See
#791 for details
- [ ] Bug fix
- [x] Code health or cleanup
- [ ] Documentation
- [ ] Other (specify)

## Testing

- [ ] **Documentation**: `make docusaurus_start`; only needed if you
make doc changes
- [ ] **Unit Tests**: `make go_develop_and_test`
- [ ] **LocalNet E2E Tests**: `make test_e2e`
- [ ] **DevNet E2E Tests**: Add the `devnet-test-e2e` label to the PR.

## Sanity Checklist

- [ ] I have tested my changes using the available tooling
- [ ] I have commented my code
- [ ] I have performed a self-review of my own code; both comments &
source code
- [ ] I create and reference any new tickets, if applicable
- [ ] I have left TODOs throughout the codebase, if applicable

proto/poktroll/service/tx.proto

@@ -73,5 +73,7 @@ message MsgAddService {
   poktroll.shared.Service service = 2 [(gogoproto.nullable) = false]; // The Service being added to the network
 }
 
-message MsgAddServiceResponse {}
+message MsgAddServiceResponse {
+  poktroll.shared.Service service = 1;
+}
 

x/service/keeper/msg_server_add_service.go

@@ -111,5 +111,7 @@ func (k msgServer) AddService(
 	k.SetService(ctx, msg.Service)
 
 	isSuccessful = true
-	return &types.MsgAddServiceResponse{}, nil
+	return &types.MsgAddServiceResponse{
+		Service: &msg.Service,
+	}, nil
 }

x/service/keeper/msg_server_add_service_test.go

@@ -43,11 +43,12 @@ func TestMsgServer_AddService(t *testing.T) {
 	keepertest.AddAccToAccMapCoins(t, oldServiceOwnerAddr, volatile.DenomuPOKT, oneUPOKTGreaterThanFee)
 
 	// Add the service to the store
-	_, err := srv.AddService(ctx, &types.MsgAddService{
+	addSvcRes, err := srv.AddService(ctx, &types.MsgAddService{
 		OwnerAddress: oldServiceOwnerAddr,
 		Service:      oldService,
 	})
 	require.NoError(t, err)
+	require.Equal(t, &oldService, addSvcRes.GetService())
 
 	// Validate the service was added
 	serviceFound, found := k.GetService(ctx, oldService.Id)