Allow adding listeners to execution

I would like to add listeners on every execution of robot test cases.

My use case:
Adding a listener that does real-time logging in to Elasticsearch.

Start by tracing how the webservice runs Robot Framework test cases and where each execution is created. Determine the listener extension point and define how callers would register a listener, then verify that execution events can support the requested real-time Elasticsearch logging use case.

Written by the indexing model from the issue text.