### 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[Response]:
        """This interceptor method is called by the proxy when returning a response
        (*after* having forwarded the request and received a response from the backend
        service). It receives details of the incoming request as well as the response
        from the backend service, and returns either of the following results:
        * An instance of requests.models.Response to return to the client instead of the
          actual response returned from the backend service.
        * Any other value, in which case the response from the backend service is
          returned to the client.
        """
        return None
    def get_forward_url(self, method: str, path: str, data, headers):
        """Return a custom URL to forward the given request to. If a falsy value is returned,
        then the default URL will be used.
        """
        return None
class MessageModifyingProxyListener(ProxyListener):