[Tech] Implement poll history cache (MSC4013) in the rust SDK

[alfogrillo]

Description

MSC4013 TL;DR;

1. Each room has a new state event m.room.poll_history
   • Ideally this event is sent when the room is created together with other initial state events (example of initial state event: m.room.encryption)
2. Every time a client starts a poll it references the m.room.poll_history state event using m.relates_to.
3. (out of scope for now) In the poll history we can call the relations API to fetch quickly all the events related with m.room.poll_history (which are the m.poll.start events). Only polls created with this approach will be visible in the history (polls already in the timeline won't be visible).

Size

M?

Dependencies

• None

Acceptance criteria

• Each new created room will has a m.room.poll_history state event
• Each new m.poll.start (including edits) event has a reference to the new m.room.poll_history state event

Out of scope

• Using the MSC for building the poll history (for now we keep the old "backward pagination" approach)

Open questions

• How can we publish the new m.room.poll_history state event in already existing rooms? Can HS admins publish it in the rooms with a script?
• Will this change prevent polls hosting threads (see comment)

Subtasks

Android

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iOS

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Other

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[jonnyandrew]

Blocked waiting on a resolution for matrix-org/matrix-spec-proposals#4013 (comment), as after this change it would not be possible to:

• start a thread on a poll
• start a poll within a thread

See internal disccusion

@VolkerJunginger, @langleyd