### 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
return os.path.join(config.dirs.cache, SERVER_CERT_PEM_FILE)
def start_proxy_server(
    port,
    bind_address=None,
    forward_url=None,
    use_ssl=None,
    update_listener: Optional[Union[ProxyListener, List[ProxyListener]]] = None,
    quiet=False,
    params=None,  
    asynchronous=True,
    check_port=True,
    max_content_length: int = None,
    send_timeout: int = None,
):
    bind_address = bind_address if bind_address else BIND_HOST
    if update_listener is None:
        listeners = []
    elif isinstance(update_listener, list):
        listeners = update_listener
    else:
        listeners = [update_listener]
    def handler(request, data):