feat(httpx): use cf-connecting-ip header for client IP

[mfzl]

Adds Cf-Connecting-IP to the list of headers to check for real client IP. This header is set by Cloudflare to have the real client IP address.

https://developers.cloudflare.com/fundamentals/reference/http-request-headers/#cf-connecting-ip

Since we are already relying on cloudflare headers for device location and how ubiquitous Cloudflare has become it's a good idea to have support for it.

We cannot rely on X-forwarded-for when behind Cloudflare as well, since the order of the IPs listed by Cloudflare is not the order this library checks.

Related Issue or Design Document

Checklist

• I have read the contributing guidelines and signed the CLA.
• I have referenced an issue containing the design document if my change introduces a new feature.
• I have read the security policy.
• I confirm that this pull request does not address a security vulnerability.
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  I confirm that I got approval (please contact [email protected]) from the maintainers to push the changes.
• I have added tests that prove my fix is effective or that my feature works.
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Further comments

[mfzl]

I could potentially also "fix" the x-forwarded-for order as well. It looks like a bug after reading this.

It should be the first public IP address going from left-to-right. Right now it's the first public IP address from right-to-left

[aeneasr]

I could potentially also "fix" the x-forwarded-for order as well. It looks like a bug after reading this.

It should be the first public IP address going from left-to-right. Right now it's the first public IP address from right-to-left

Yes please - if there is a bug we appreciate a fix!

[mfzl]

I could potentially also "fix" the x-forwarded-for order as well. It looks like a bug after reading this.
It should be the first public IP address going from left-to-right. Right now it's the first public IP address from right-to-left

Yes please - if there is a bug we appreciate a fix!

Hey @aeneasr I will raise another PR for that. If you have no objections to the changes to the PR this can be merged and closed

[aeneasr]

Sounds good!