Support AWS Lambda@Edge

**Is your feature request related to a problem? Please describe.**
I want to use Elastic APM in AWS Lambda@Edge functions. However [AWS Lambda@Edge does not support layers and environment variables](https://docs.aws.amazon.com/AmazonCloudFront/latest/DeveloperGuide/edge-functions-restrictions.html#lambda-at-edge-function-restrictions). I can use Elastic APM in regular AWS Lambda functions [by following the instructions](https://www.elastic.co/guide/en/apm/agent/nodejs/current/lambda.html)

**Describe the solution you'd like**
Instructions on how to bundle the extension inside ZIP, webpack the client library, and manually initialize the APM client would be appreciated.

**Describe alternatives you've considered**
None.

**Additional context**
We would like to measure the performance of our Lambda@Edge functions.