### 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
DEFAULT_LISTENERS = []
    def forward_request(
        self, method: str, path: str, data: MessagePayload, headers: Headers
    ) -> Union[int, Response, Request, dict, bool]:
        """This interceptor method is called by the proxy when receiving a new request
        (*before* forwarding the request to the backend service). It receives details
        of the incoming request, and returns either of the following results:
        * True if the request should be forwarded to the backend service as-is (default).
        * An integer (e.g., 200) status code to return directly to the client without
          calling the backend service.
        * An instance of requests.models.Response to return directly to the client without
          calling the backend service.
        * An instance of requests.models.Request which represents a new/modified request
          that will be forwarded to the backend service.
        * Any other value, in which case a 503 Bad Gateway is returned to the client
          without calling the backend service.
        """
        return True
    def return_response(