### Is there an existing issue for this?

- [X] I have searched the existing issues

### Current Behavior

We are using the `Authorization` header for routing purposes

```
 "authorization: AWS4-HMAC-SHA256 <...>"
```

https://github.com/localstack/localstack/issues/5770

### Expected Behavior

Use a custom header to avoid causing confusion with backend integrations

### How are you starting LocalStack?

With a `docker run` command

### Steps To Reproduce

Run a custom HTTP integration without authorisation

### Environment

```markdown
- OS: 
- LocalStack:
```


### Anything else?

No
import gzip
import json
import logging
import os
import re
import subprocess
import sys
import threading
from typing import Dict, Optional
from requests.models import Response
from localstack import config
from localstack.constants import (
    HEADER_LOCALSTACK_EDGE_URL,
    HEADER_LOCALSTACK_REQUEST_URL,
    INTERNAL_AWS_ACCESS_KEY_ID,
    LOCALHOST,
    LOCALHOST_IP,
    LOCALSTACK_ROOT_FOLDER,
    LS_LOG_TRACE_INTERNAL,
    PATH_USER_REQUEST,
)
from localstack.http import Router
from localstack.http.dispatcher import Handler, handler_dispatcher
from localstack.http.router import RegexConverter
from localstack.runtime import events
from localstack.services.generic_proxy import ProxyListener, modify_and_forward, start_proxy_server