Fix event type & namespace filters for external streaming destinations backend

What does this MR do and why?

Fix namespace filters for external streaming destinations

Also adds support for instance-level namespace filters in the backend in doing so

I noticed this while working on the frontend to add Instance namespace filters to the frontend: !197028 (merged)

References

Screenshots or screen recordings

Before	After
Screen_Recording_2025-07-16_at_5.14.03_PM	Screen_Recording_2025-07-16_at_5.16.41_PM

How to set up and validate locally

• git checkout master
• Enable feature flags: audit_events_external_destination_streamer_consolidation_refactor use_consolidated_audit_event_stream_dest_api if not already enabled (default is true)
• Go to a Group and create a HTTP / other streaming destination (can use something like this: https://webhook.site/)
• Filter the namespace by group or project -- right now master does not filter it properly
• Check this branch out in git
• Re-do the test, it should only emit relevant audit events to the namespace
• Test in combination with event type filters

MR acceptance checklist

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#557593 (closed)