Private read receipts support (msc 2285)

[symphorien]

Is your feature request related to a problem? Please describe.
On other devices, I use clients that support msc 2285, so send private read receipts. Schlidichat ignores those, so rooms that I have read on other devices show as unread on schlidichat.
This is besides the obvious privacy benefit of private read receipts.
This option is standard and different from the "open without reading" option in that

• it is automatic
• the read marker is moved
• the read marker is shared across clients

Describe the solution you'd like
Support for msc 2285 matrix-org/matrix-spec-proposals#2285