[Feature] - Allow cloudflare sources in firewall rules

[mo36924]

Is your feature request related to a problem? Please describe. Vultr load balancer allows you to specify "cloudflare" as the firewall source, but vke allows you to specify only cidr as the firewall source. https://github.com/vultr/vultr-cloud-controller-manager/blob/v0.6.0/vultr/loadbalancers.go#L736

Describe the solution you'd like Like the Vultr load balancer, allow cloudflare to be the source of vke firewall rules.