media_retention - enhancement to provide more control over deletion.

This issue has been migrated from [#15434](https://github.com/matrix-org/synapse/issues/15434).

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**Enhance features to purge media regardless of when it was accessed last**

Media retention limits are observed, as long as the file is not accessed. This is clearly stated here:

`options control whether media will be purged if it has not been accessed in a given amount of time. Note that media is 'accessed' when loaded in a room in a client, or otherwise downloaded by a local or remote user.`
https://matrix-org.github.io/synapse/latest/usage/configuration/config_documentation.html#media_retention

That's awesome. But as long as someone somewhere on some random federated server keeps hitting up the file, it will remain forever. Could an override be put in to delete media after a set period, regardless of when it was accessed last?
One caveat, it would need to not apply to Profile pictures and room profile pictures.