[sr-dev] [kamailio/kamailio] Proxy Protocol support (#1757)

[Sebastian Lauwers]

@miconda I would mainly love some pointers as to how/where to implement this.

My initial thought was to modify `handle_new_connect()` to change the connection state to `S_CONN_OK` instead of `S_CONN_ACCEPT`, and then peek the connection to see if we are getting a PROXY v1/v2 header, and the override the connection information structs. However, this obviously runs amuck of clean code, and obviously might wait a long time if the header isn't sent immediately.

Would it, instead, make more sense to attempt to parse it in a fashion not unlike the `tcp_read_hep3`/`tcp_header_headers` combination in `tcp_read.c` around line 1490:

```c
if(unlikely(ksr_tcp_accept_hep3!=0)) {
  bytes=tcp_read_hep3(con, read_flags);
  if (bytes>=0) {
    if(!(con->req.flags & F_TCP_REQ_HEP3)) {
      /* not hep3, try to read headers */
      bytes=tcp_read_headers(con, read_flags);
    }
  }
} else {
  bytes=tcp_read_headers(con, read_flags);
}
```

So for example:

```c
if(unlikely(ksr_tcp_accept_hep3!=0)) {
  bytes=tcp_read_hep3(con, read_flags);
  if (bytes>=0) {
    if(!(con->req.flags & F_TCP_REQ_HEP3)) {
      /* not hep3, try to read headers */
      goto read_headers
    }
  }
} else {
read_headers:
  if (unlikely(read_proxy_protocol!=0)) {
    tcp_read_proxy_protocol(con, read_flags);
  }
  bytes=tcp_read_headers(con, read_flags);
}
```

Obviously, the above is just imaginary pseudocode. Thoughts?

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