[Logs Overview] Expose categorization service on the start contract of logs_shared

:notebook: Summary

To avoid duplication (and divergence) of code-paths we want to expose the categorization service for other consumers.

A categorization service is exposed on the start contract of the logs_shared plugin on both the browser and server side.
The categorization service encapsulates the functionality of the state machine that is currently used to categorize logs in the logs overview.
The logs overview also uses that shared service implementation.
The categorization service supports additional parameters to let the consumer optionally tweak the similarity threshold and min doc count of the first categorization pass.
The LogCategory result data structure includes an example message obtained via top_hits for each category.