Could we decouple App Insights from Script to allow for independently updating versions?

Since all of our logging goes through ILogger, we could theoretically remove any direct dependencies on App Insights, which may make it easier for people to independently move their App Insights versions forward. Theoretically, I think we could do this but it'd require a couple of things -- a way to construct the LoggerFactory (covered by #1579), and a way to keep this simple to configure (like how we have a 'simple magic setting' today that turns it all on).

The idea came from @TheeJamesLee in https://github.com/Azure/azure-webjobs-sdk/pull/1220, so I wanted to track it here independently.

Will require some investigation as to whether it'd be feasible (and how many people would use it).