### 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
def __init__(self, base_url: str, forward_url: str) -> None:
        super().__init__()
        self.forwarder = UrlMatchingForwarder(
            base_url=base_url,
            forward_url=forward_url,
        )
    def forward_request(self, method, path, data, headers):
        return self.forwarder.forward_request(method, path, data, headers)
    def register(self):
        ProxyListener.DEFAULT_LISTENERS.append(self)
    def unregister(self):
        try:
            ProxyListener.DEFAULT_LISTENERS.remove(self)
        except ValueError:
            pass
class FakeEndpointProxyServer(Server):
    """
    Makes an EndpointProxy behave like a Server. You can use this to create transparent
    multiplexing behavior.
    """
    endpoint: EndpointProxy
    def __init__(self, endpoint: EndpointProxy) -> None:
        self.endpoint = endpoint
        self._shutdown_event = threading.Event()
        self._url = self.endpoint.forwarder.base_url
        super().__init__(self._url.port, self._url.hostname)
    @property
    def url(self):