Add hook for logs

[ejizba]

This PR provides a clean way to perform an operation every time the worker sends a log. The motivation behind this is to send each log to open telemetry (tracked by Azure/azure-functions-nodejs-library#245). Most of the otel support will be in a separate library, and I have a working prototype I can demo offline, but nothing ready for a legit PR yet. The library can be finalized at a later date, but I want to get these worker changes in so they can start rolling out.

There is a workaround available to the library today meaning these changes aren't strictly necessary, but it has two drawbacks:

1. It's ugly (as described here)
2. It only lets you access user logs and this new hook lets you access both user and system logs