feat(Core/Network): Add Proxy Protocol v2 support.

[walkline]

In some cases, you may want to place the authserver or worldserver behind a proxy or load balancer. However, if you do this, you would lose the client's address (IP). To address this issue, most proxies and load balancers support Proxy Protocol (v1 & v2). To utilize it, we also need Proxy Protocol support on both the authserver and worldserver.

Changes Proposed:

This PR proposes changes to:

• Core (units, players, creatures, game systems).
• Scripts (bosses, spell scripts, creature scripts).
• Database (SAI, creatures, etc).

Issues Addressed:

• Closes Feature: Network support proxy protocol #18100

SOURCE:

The changes have been validated through:

• Live research (checked on live servers, e.g Classic WotLK, Retail, etc.)
• Sniffs (remember to share them with the open source community!)
• Video evidence, knowledge databases or other public sources (e.g forums, Wowhead, etc.)
• The changes promoted by this pull request come partially or entirely from another project (cherry-pick). Cherry-picks must be committed using the proper --author tag in order to be accepted, thus crediting the original authors, unless otherwise unable to be found

Tests Performed:

This PR has been:

• Tested in-game by the author.
• Tested in-game by other community members/someone else other than the author/has been live on production servers.
• This pull request requires further testing and may have edge cases to be tested.

How to Test the Changes:

I used HAProxy to test this change.

1. I changed authserver and worldserver configs by changing ports (3777 & 8026) and by enabling Proxy Protocol.
2. Run HAProxy with the next config:

global
    log /dev/log    local0
    log /dev/log    local1 notice
    stats timeout 30s

defaults
    log global
    mode tcp
    option tcplog
    timeout connect 5000ms
    timeout client 50000ms
    timeout server 50000ms

frontend tcp_front
    bind 0.0.0.0:3724
    default_backend tcp_back

frontend tcp_world
    bind 0.0.0.0:8085
    default_backend tcp_world_back

backend tcp_back
    server backend_server 127.0.0.1:3777 send-proxy-v2

backend tcp_world_back
    server backend_server 127.0.0.1:8026 send-proxy-v2

3. Login to the game using regular ports.
4. You can use .lookup ip command to verify players IP.

Known Issues and TODO List:

• [ ]
• [ ]

How to Test AzerothCore PRs

When a PR is ready to be tested, it will be marked as [WAITING TO BE TESTED].

You can help by testing PRs and writing your feedback here on the PR's page on GitHub. Follow the instructions here:

http://www.azerothcore.org/wiki/How-to-test-a-PR

REMEMBER: when testing a PR that changes something generic (i.e. a part of code that handles more than one specific thing), the tester should not only check that the PR does its job (e.g. fixing spell XXX) but especially check that the PR does not cause any regression (i.e. introducing new bugs).

For example: if a PR fixes spell X by changing a part of code that handles spells X, Y, and Z, we should not only test X, but we should test Y and Z as well.