Warn users about next purge and let them download all their data that will be purged

This issue has been migrated from [#15704](https://github.com/matrix-org/synapse/issues/15704).

---

**Description:**

When a synapse admin is planning to purge history older than x years (month...), it would be nice if they could warn all users of the server when this purge will occur to give them time to download data they want to keep.

It would be even better if the user could download either all his/her data that will be purged (older than x year... but not the recent one that won't be purged) or to be able to do it per room (maybe the user don't care to loose old data from certain rooms).

That could take the form of a private read only room where admin send message to the user with link to download the data that will soon be purged (one link to download totally and one link per room)

No files, tests, or entry points are identified. Start by clarifying the purge-warning flow, the data and room-level download scope, and the delivery mechanism; done should include an agreed design for notifying users and exporting only data scheduled for purge.

Written by the indexing model from the issue text.