### 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_
self, xmlnode: ETree.Element, params: Union[str, bytes], _, name: str
    ) -> None:
        node = ETree.SubElement(xmlnode, name)
        node.text = self._get_base64(params)
    def _serialize_type_timestamp(
        self, xmlnode: ETree.Element, params: str, shape: Shape, name: str
    ) -> None:
        node = ETree.SubElement(xmlnode, name)
        node.text = self._convert_timestamp_to_str(
            params, shape.serialization.get("timestampFormat")
        )
    def _default_serialize(self, xmlnode: ETree.Element, params: str, _, name: str) -> None:
        node = ETree.SubElement(xmlnode, name)
        node.text = str(params)
    def _pional_traelf, rootree.Element]):
        """
        Prepares the XML root node before being serialized with additional traits (like the Response ID in the Query
        protocol).
        For some protocols (like rest-xml), the root can be None.
        """
        pass
    def _create_default_response(self, operation_model: OperationModel) -> HttpResponse:
        response = super()._create_default_response(operation_model)
        response.headers["Content-Type"] = "text/xml"
        return response
    def _xml_to_string(self, rootree.ElementTree]) -> Optional[str]: