Add support for the "Purge History" API

[RealOrangeOne]

When managing rooms, it's useful to be able to purge the room history to reduce storage consumed by Synapse.

It'd be great if it were possible to do this through synapse-admin.

See https://matrix-org.github.io/synapse/latest/admin_api/purge_history_api.html for API reference

[Thatoo]

That would be awesome : https://element-hq.github.io/synapse/latest/admin_api/purge_history_api.html

[luodaoyi]

+1