Implement `?format={event,content}` for `GET /state/:type{/:key,}`

This is not in the spec but it's considered an omission rather than a new feature.

• Requested by: @emma:conduit.rory.gay here
• Spec issue: https://github.com/matrix-org/matrix-spec/issues/1047
• Synapse implementation: https://github.com/element-hq/synapse/blob/v1.108.0/synapse/rest/client/room.py#L244-L271