Rate Limit /64 subnets when remote address is IPv6

[csarn]

In https://docs.rs/actix-ratelimit/latest/src/actix_ratelimit/middleware.rs.html#62-76, the default identifier for a client is its ip address.
But IPv6 clients usually get at least a /64 assigned to them, so a single machine could easily exhaust memory for the rate-limit store and/or avoid being rate-limited by rotating through ip addresses within its /64.
See also: https://adam-p.ca/blog/2022/02/ipv6-rate-limiting/

I suggest to extract the /64 subnet in the default identifier and thus rate-limit the complete subnet.