Idea: Refactor executive to support alternate execution

[eoware]

The current implementation does the config, startup, and event watching in the program class/main method. To better support other implementations like an asp.net core process, a service class could be refactored out of program that could be started from any app/process. It would probably also make sense to move that refactored service and supporting code to Puppet.Common so other app types could avoid referencing Puppet.Executive.