Revisit logging extensibility

Under .NET core we no longer support per add-in configuration files, which makes logging configuration difficult for the Excel-DNA built-in logging that runs before the user add-in code is loaded.

We have added some registry settings that will cause the excel DNA trace logging to be written to a file or the debugger - see https://excel-dna.net/docs/guides-advanced/logging_configuration_netcore/

However, it might be nice to have some extensibility mechanism that allows the add-in to configure or load other logging mechanisms, e.g. logging to NLog - see this request https://groups.google.com/g/exceldna/c/rgNjoQZMzzU

Also follows on https://github.com/Excel-DNA/ExcelDna/issues/277