### 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 forward_request(
        self, method: str, path: str, data: MessagePayload, headers: Headers
    ) -> Optional[RoutingRequest]:
        """Return a RoutingRequest with modified request data, or None to forward the request
        unmodified"""
        return None
    def return_response(