File Source Improvements and update to Event Model

[graytaylor0]

The file source is a good way to get started and test the grok prepper. The recent
Event model change #538 broke the connection between the fileand grok, as it still uses a Record<String> type while grok was updated to a Record<Event>

Additionally, the file source is useful for those who just want to run data prepper on an application host, and this issue will also involve refactoring the file source and start to add some improvements to it.