### 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, service_manager=None) -> None:
        super().__init__()
        self.service_manager = service_manager or SERVICE_PLUGINS
    def forward_request(self, method, path, data, headers):
        
        headers.get(HEADER_KILL_SIGNAL) and sys.exit(0)
        if events.infra_stopping.is_set():
            return 503
        if config.EDGE_FORWARD_URL:
            return do_forward_request_network(
                0, method, path, data, headers, target_url=config.EDGE_FORWARD_URL
            )