Add Elastic Application Performance Monitoring (APM) extension

[mattrenner]

Elastic APM is a performance monitoring solution that has an agent / SDK for instrumenting multiple different languages, including PHP.

We've been using this for a few months successfully in a production environment so figure we should share the work that was done to allow adding this to Bref.

This PR adds the elastic-apm extension, along with some brief documentation on how to configure it with the experimental Lambda Extension also provided by Elastic.