Create audit event external streamer classes

What does this MR do and why?

Consolidate audit event streaming strategies to streaming destinations'

Follow-up MR to: !171429 (merged)

We have audit event external streaming destinations, and this MR introduces a Streamer structure, the bulk of the work is found in the BaseStreamer which consolidates the logic how to stream the event audit events by calling on the destinations.

There is also two files, group/streamer.rb and instance/streamer.rb which define streamable? and destinations as they are different on how they are fetched or determined valid.

With this MR, there is no caller for either the Streamer or Destinations, which will happen behind a feature flag in this MR: !171431 (merged)

Changelog: added EE: true

References

• Resolves part of: #436616 (closed)
• Resolves part of: #436617 (closed)

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