Allow setting of trusted CIDR blocks for X-Forwarded-For

[robd003]

Description:
Last week Envoy just released support for trusted CIDR blocks when processing X-Forwarded-For headers.

It would be really useful to have a default configuration mode called "cloudflare" which would automatically load the IP ranges from these two dynamically updated lists:

Cloudflare IPv4 CIDR blocks: https://www.cloudflare.com/ips-v4/
Cloudflare IPv6 CIDR blocks: https://www.cloudflare.com/ips-v6/

[optional Relevant Links:]
envoyproxy/envoy#31831
https://github.com/envoyproxy/envoy/releases/tag/v1.32.0

[zhaohuabing]

A knob for configuring CIDR can be added to the ClientTrafficPolicySpec.ClientIPDetection.XForwardedForSettings,

NumTrustedHops: 2
TrustedCIDRs:
  - 173.245.48.0/20
  - 103.21.244.0/22

Given that the IP lists are updated only once a year, I believe it’s reasonable to just manually update the CTP whenever Cloudflare makes changes. cc @envoyproxy/gateway-maintainers

[arkodg]

+1 to a generic trustedCIDRs

[rudrakhp]

@arkodg I would like to contribute to this, please assign if help wanted. Thanks!