Add config option to disable sending read receipts to other users.

This issue has been migrated from [#9253](https://github.com/matrix-org/synapse/issues/9253).

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Hello,

I'm implementing Synapse for a public sector entity where the worker's council is likely to enforce enhanced privacy for message read receipts. As far as I understood Synapse's event handling, this isn't easy to accomplish in the current state of implementation.

I'm currently looking at pseudonymizing read receipt events for a public room as a workaround (others, like private rooms, might be harder to implement and will likely not be requested here in the first place). Is it possible to overwrite events in the database, i.e. so that every m.read or m.fully_read is then related to some dummy user (e.g. a service account)? People would see 'someone' read the message but can't tell who it was anymore.