### Is there an existing issue for this?

- [X] I have searched the existing issues

### Current Behavior

When attempting to sign some document using Localstack KMS inside a lambda container (also running on localstack), the localstack KMS listener returns some unknown error.

The KMS key and lambda containers have been created from the same CDK app deploy, and I can see all the resources in the localstack web console.

The test application I'm using to prove this behaviour is available here:
https://github.com/LukaHedt/localstack_kms_test

Instructions to run are in the main Readme, and the `/localstack/Readme.md` file.

`aws-sdk` returns an unknown error, and the localstack docker logs that I receive are available here:
https://pastebin.com/58Z3fXi3

Have done an amount of testing and can confidently claim the following:
- Have tested on multiple machines with the same outcome
- Doesn’t make a difference if I specify an ARN or a KeyID in the signing request
- Downgrading localstack versions either didn’t make a difference, or I couldn’t actually deploy a localstack container with version 0.12.20 or lower.

### Expected Behavior

KMS Sign should return a signature for the given payload.

### How are you starting LocalStack?

With a docker-compose file

### Steps To Reproduce

Test application demonstrating the current behaviour is available here:

https://github.com/LukaHedt/localstack_kms_test

Full instructions to run are in the `/localstack/Readme.md` file.


### Environment

```markdown
- OS: MacOS Monteray 12.3
- LocalStack: 0.14.0
```


### Anything else?

_No response_
The ible for the serialization of responses from services which use the
    ``ec2`` protocol (basically the EC2 service). This protocol is basically equal to the ``query`` protocol with only
    a few subtle differences.
    **Experimental:** This serializer is still experimental.
    When implementing services with this serializer, some edge cases might not work out-of-the-box.
    """
    def _serialize_error(
        self,
        error: ServiceException,
        code: str,
        sender_fault: bool,
        response: HttpResponse,
        shape: Shape,
        operation_model: OperationModel,
    ) -> None:
        # EC2 errors look like:
        # <Response>
        #   <Errors>
        #     <Error>
        #       <Code>InvalidInstanceID.Malformed</Code>
        #       <Message>Invalid id: "1343124"</Message>
        #     </Error>
        #   </Errors>
        #   <RequestID>12345</RequestID>
        # </Response>
        # This is different from QueryParser in that it's RequestID, not RequestId
        # and that the Error tag is in an enclosing Errors tag.
        attr = (
            {"xmlns": operation_model.metadata.get("xmlNamespace")}
            if "xmlNamespace" in operation_model.metadata