Make invocation timeouts visible in Elastic APM

When a lambda invocation times out, then in most cases the APM agents won't be able to finalize and send the corresponding transaction to the extension and APM server. As a consequence, lambda function invocations that time out are not visible in APM at all. However, seeing timed out invocations in APM is essential as it is an important indicator that something is wrong with a function or it is too slow.

The lambda runtime [notifies external extensions](https://docs.aws.amazon.com/lambda/latest/dg/runtimes-extensions-api.html) with the `shutdown` event in case of a timeout. This mechanism could be used by the APM extension to generate and send an artificial transaction (with a `failure` state and additional information on the timeout). In this way APM would provide information on timed out invocations.