fix: handle the side-effect artifacts case

[joeyzhao2018]

What does this PR do?

Handle the case where the original handler code returns a side-effect artifact. Such as aws-serverless-express server.

Motivation

https://datadoghq.atlassian.net/browse/SLES-2468

Testing Guidelines

1. Added Unittests
2. Tested with a real lambda application

Additional Notes

Types of Changes

• Bug fix
• New feature
• Breaking change
• Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• This PR's description is comprehensive
• This PR contains breaking changes that are documented in the description
• This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• This PR impacts documentation, and it has been updated (or a ticket has been logged)
• This PR's changes are covered by the automated tests
• This PR collects user input/sensitive content into Datadog
• This PR passes the integration tests (ask a Datadog member to run the tests)

[duncanista]

This change might need to also be checked if needed in dd-trace-js integration

[joeyzhao2018]

where in dd-trace-js?

[duncanista]

https://github.com/DataDog/dd-trace-js/blob/master/packages/dd-trace/src/lambda/handler.js

[joeyzhao2018]

That patching is safe. It's because it is yet another layer of wrapping above the patching done here in this repo.
My understanding is that this promisifiedHandler function exists in order to make it easier for the wrapping to add pre and post run handling, i.e., enabling it to use try { await the_promisifiedHandler} finally {}. And it just need to be done once. So dd-trace-js part should be good.