### 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
"""Construct a URL from the given base URL and path"""
    parsed = urlparse(base_url)
    path = path or ""
    path = path if path.startswith("/") else f"/{path}"
    protocol = f"{parsed.scheme}:" if parsed.scheme else ""
    return f"{protocol}//{parsed.netloc}{path}"
def with_context():
    """
    Decorator wraps function in a request context manager
    :return:
    """
    def context_manager(method=None, path=None, data_bytes=None, headers=None, *args, **kwargs):
        req_context = get_proxy_request_for_thread()
        ctx_manager = empty_context_manager()
        if not req_context:
            req_context = Request(url=path, data=data_bytes, headers=headers, method=method)
            ctx_manager = RequestContextManager(req_context)
        return ctx_manager
    def decorator(func):
        @functools.wraps(func)
        def wrapper(*args, **kwargs):
            ctx_manager = context_manager(*args, **kwargs)
            with ctx_manager:
                value = func(*args, **kwargs)
            return value
        return wrapper
    return decorator
@with_context()
def modify_and_forward(