Add new "System Only" ILogger capability

Many of the logs the Host writes are really intended or only useful for Azure Functions service team members. Such logs are just noise for customers if they go to their log streams, increasing costs for them.

So we want to have the ability for platform components in the host to request and use a "system only" ILogger.

No file, test, or entry point is named. Start by locating the host's ILogger creation and the platform components that emit these logs, then determine the intended system-only behavior and its effect on customer log streams. Done means the capability is defined, integrated for platform logging, and covered by tests.

Written by the indexing model from the issue text.