When we return events to clients we need to annotate them with the
membership of the user at the time of the event, in the `unsigned`
section. We already check the membership at the event during the
visibility checks, and so we annotate events there. However, since this
a per-user field we end up having to clone the event in question.
Instead, let's add a `FilteredEvent` class that is returned by the
visibility checks, which allows returning the membership without editing
the event. This has three benefits:
1. Avoids the clones of the event.
2. Allows us to statically check that we have filtered events before
returning them to clients.
3. We no longer edit `unsigned` data after event deserialization, this
makes it easier to port the event class to Rust.
The last benefit is why we're doing this *now*, however IMV it shouldn't
affect whether we want this change or not.
Reviewable commit-by-commit
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Co-authored-by: Olivier 'reivilibre' <oliverw@element.io>
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