Switch to chunk events so we can auth via power_levels (MSC2716)

[MadLittleMods]

Switch to chunk events so we can auth via power_levels in #10245

Previously, we were using content.chunk_id to connect one
chunk to another. But these events can be from any sender
and we can't tell who should be able to send historical events.
We know we only want the application service to do it but these
events have the sender of a real historical message, not the
application service user ID as the sender. Other federated homeservers
also have no indicator which senders are an application service on
the originating homeserver.

So we want to auth all of the MSC2716 events (like insertion, chunk, marker) via power_levels
and have them be sent by the application service with proper
PL levels in the room.

Before	After

Mermaid live editor playground link

mermaid graph syntax

flowchart BT
    subgraph live
        B --------------> A
    end
    
    subgraph chunk0
        chunk0-chunk[["chunk"]] --> chunk0-2(("2")) --> chunk0-1((1)) --> chunk0-0((0)) --> chunk0-insertion[/insertion\]
    end

    subgraph chunk1
        chunk1-chunk[["chunk"]] --> chunk1-2(("2")) --> chunk1-1((1)) --> chunk1-0((0)) --> chunk1-insertion[/insertion\]
    end
    
    subgraph chunk2
        chunk2-chunk[["chunk"]] --> chunk2-2(("2")) --> chunk2-1((1)) --> chunk2-0((0)) --> chunk2-insertion[/insertion\]
    end

    
    chunk0-insertionBase[/insertion\] -------> A
    chunk0-chunk -.-> chunk0-insertionBase[/insertion\]
    chunk0-insertion -------> A
    chunk1-insertion --> A
    chunk1-chunk -.-> chunk0-insertion
    chunk2-insertion --> A
    chunk2-chunk -.-> chunk1-insertion

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TARDIS visualization of the DAG

Pull Request Checklist

• Pull request is based on the d)evelop branch
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  • Use markdown where necessary, mostly for code blocks.
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• Code style is correct (run the linters)

[MadLittleMods]

Feel free to suggest better names!

Maybe we want to rename insertion to chunk_end so we have chunk_start and chunk_end events 🤷‍♀️ (which one is the start vs end 😜, depends on frame of reference)

[erikjohnston]

woo

[MadLittleMods]

Thanks @erikjohnston 🐫