### 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
data: Union[Response, bytes],
    templates: Dict[str, str],
    content_type: str = None,
    as_json: bool = False,
):
    """Apply the matching request/response template (if it exists) to the payload data and return the result"""
    content_type = content_type or APPLICATION_JSON
    is_response = isinstance(data, Response)
    templates = templates or {}
    template = templates.get(content_type)
    if not template:
        return data
    content = (data.content if is_response else data) or ""
    result = VtlTemplate().render_vtl(template, content, as_json=as_json)
    if is_response:
        data._content = result
        update_content_length(data)
        return data
    return result
UPDATE_APIGATEWAY = ProxyListenerApiGateway()