### 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
self,
        method: str,
        path: str,
        data: MessagePayload,
        headers: Headers,
        response: Response,
    ) -> Optional[RoutingResponse]:
        
        if is_internal_call_context(headers):
            return None
        return RoutingResponse(
            status_code=response.status_code,
            content=self._adjust_partition(response.content),
            headers=self._adjust_partition(response.headers),
        )
    def _adjust_partition_in_path(self, path: str, static_partition: str = None):
        """Adjusts the (still url encoded) URL path"""
        parsed_url = urlparse(path)
        
        
        decoded_query = parse_qs(qs=parsed_url.query, keep_blank_values=True)
        adjusted_path = self._adjust_partition(parsed_url.path, static_partition)
        adjusted_query = self._adjust_partition(decoded_query, static_partition)
        encoded_query = urlencode(adjusted_query, doseq=True)
        
        encoded_query = encoded_query.replace("=&", "&")
        encoded_query = re.sub(r"=$", "", encoded_query)
        return f"{adjusted_path}{('?' + encoded_query) if encoded_query else ''}"
    def _adjust_partition(self, source, static_partition: str = None):