MSC4446: Allow moving the fully read marker to older events

[SpiritCroc]

Rendered

[tulir]

Implementation requirements:

• Server that ignores attempts to roll back without the flag
• Server that allows rolling back with the flag
• Client that uses the flag to intentionally roll back m.fully_read
• Testing to ensure old clients don't explode when m.fully_read rolls back

[SpiritCroc]

As mentioned in comments further down, I can currently offer:

• Server implementation via synapse PR (pending review): Support MSC4446: allow moving fully read markers backwards element-hq/synapse#19663
• Client implementation via SchildiChat Revenge / Rust SDK: SchildiChat/matrix-rust-sdk@e2d4ec2

Furthermore, here some clients that didn't explode on initial testing yet when rolling the marker back:

• SchildiChat Next (Element X Android base)
• SchildiChat Android Legacy (Element Android Classic base)
• SchildiChat Web (Element Web base, though on somewhat older 1.11.112 version)
• Beeper Android & Desktop (at the time of writing not implementing this MSC yet)

[SpiritCroc]

Synapse PR / server implementation: element-hq/synapse#19663

[SpiritCroc]

SchildiChat Revenge implements this via SchildiChat/matrix-rust-sdk@e2d4ec2 / SchildiChat/ruma@54b2407 (client side code already allowed picking any message for setting the fully read marker, so only SDK-side usage of the flag was needed).

In practice the client currently only invokes the /receipt endpoint for this purpose, not /read_markers. I don't have a strong opinion on whether both endpoints should allow the new flags, but figured for consistency it makes sense to allow for both, to support clients which may want to prefer /read_markers over /receipt.