### 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
"""An exception indicating that a region could not be matched to a partition."""
        pass
    arn_regex = re.compile(
        r"arn:"  
        r"(?P<Partition>(aws|aws-cn|aws-iso|aws-iso-b|aws-us-gov)*):"  
        r"(?P<Service>[\w-]*):"  
        r"(?P<Region>[\w-]*):"  
        r"(?P<AccountID>[\w-]*):"  
        r"(?P<ResourcePath>"  
        r"((?P<ResourceType>[\w-]*)[:/])?"  
        r"(?P<ResourceID>[\w\-/*]*)"  
        r")"
    )
    def forward_request(
        self, method: str, path: str, data: MessagePayload, headers: Headers
    ) -> Optional[RoutingRequest]:
        return RoutingRequest(
            method=method,
            path=self._adjust_partition_in_path(path, self.DEFAULT_INBOUND_PARTITION),
            data=self._adjust_partition(data, self.DEFAULT_INBOUND_PARTITION),
            headers=self._adjust_partition(headers, self.DEFAULT_INBOUND_PARTITION),
        )
    def return_response(