Option for disabling the IP Logging

This issue has been migrated from [#8473](https://github.com/matrix-org/synapse/issues/8473).

---

Currently there is no option which allows us to disable the storing of users IP addresses in the database.

There is the config `user_ips_max_age` which helps removing the IP's after a short timeframe and it is possible to do something in the reverse proxy like replacing the `X-Forwarded-For` header with something arbitrary like `0.0.0.0`, but this also disables things like IP based rate limiting.

From a data privacy perspective I think there should be an option for disabling the storing of user IP's at all.

I'm personally motivated to work on that, but I'm also quite unfamiliar with synapse..
so if this feature is wished and anyone can give me some hints how this would be implemented in the best possible way, I probably could do it! :)