builtins: add `net.cidr_is_valid` for validating CIDR strings (#5306)

This commit adds the `net.cidr_is_valid` builtin, which makes
validating network CIDR strings much easier in policies.

Example policy:

    allow {
        net.cidr_is_valid("192.168.0.0/24")
    }

This builtin works for both IPv4 and IPv6 CIDR strings.

Signed-off-by: Ricardo Maraschini <ricardo.maraschini@gmail.com>