feat: Implemented support for X-Forwarded-For header created by jghaanstra and hajimAIM

[onno204]

Description

This PR implements X-Forwarded-For & Cf-Connecting-Ip headers to support the IP Rules of Pangolin.

All credits go to @hajimAIM for creating an IPv4 version, and to @jghaanstra for updating it to also support IPv6.
Since both their repositories are forked incorrectly, I was unable to create a PR from their repositories.

How to test?

To test, set up a Pangolin instance behind a proxy and use X-Forwarded-For headers.
Update Traefik to trust the Proxy in the traefik config file:

entryPoints:
  web:
    address: ":80"
    forwardedHeaders:
      trustedIPs:
        - "10.10.10.10/32"
  websecure:
    address: ":443"
    forwardedHeaders:
      trustedIPs:
        - "10.10.10.10/32"

Next, us the IP rules in pangolin to block/allow access as shown in the screenshot.

[onno204]

This resolves issue #6

[marcschaeferger]

@onno204 Thanks!

Could you please adjust two things to make it easier to integrate your change?

1. Open the PR against the dev branch instead of main.
2. Check the PR I submitted to your repository so it incorporates the upcoming improvements from Badger:
   Update to refactored Code of Badger onno204/badger#1