### 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
response._content = json.dumps(json_safe(listener_result))
            response.headers["Content-Type"] = APPLICATION_JSON
            response.status_code = 200
            break
        elif isinstance(listener_result, Request):
            
            
            modified_request_to_backend = listener_result
            break
        elif http2_server.get_async_generator_result(listener_result):
            return listener_result
        elif listener_result is not True:
            
            code = listener_result if isinstance(listener_result, int) else 503
            response = Response()
            response.status_code = code
            response._content = ""
            response.headers["Content-Length"] = "0"
            append_cors_headers(request_headers=headers, response=response)
            return response
    
    headers_to_send = None
    data_to_send = None
    method_to_send = None
    if response is None:
        headers_to_send = handler_chain_request.headers
        headers_to_send["Connection"] = headers_to_send.get("Connection") or "close"
        data_to_send = handler_chain_request.data
        method_to_send = handler_chain_request.method
        request_url = get_proxy_backend_url(handler_chain_request.path, run_listeners=True)