Add a way of creating extension packages

[damienbod]

It would be nice if you could create extensions for this library so that third party extensions can be created and used, something like hw the loggers do this. Example Data extensions packages for MySQL, PostgreSQL could be created or OPC DA, XML, UA entensions as an example.

[bradwilson]

The goal right now is to have each project under src to be a separate NuGet package. Extensions will be able to leverage Microsoft.Extensions.HealthChecks as the extensibility point. Please feel free to provide feedback where extensibility points should be added!