### 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,
        parameters: dict,
        response: HttpResponse,
        shape: Optional[Shape],
        shape_members: dict,
        operation_model: OperationModel,
    ) -> None:
        """
        Serializes the given payload.
        :param parameters: The user input params
        :param response: The final serialized HttpResponse
        :param shape: Describes the expected output shape (can be None in case of an "empty" response)
        :param shape_members: The members of the output struct shape
        ed here
        :return: None - the given `serialized` dict is modified
        """
        if shape is None:
            return
        payload_member = shape.serialization.get("payload")
        if payload_member is not None and shape_members[payload_member].type_name in [
            "blob",
            "string",
        ]:
            # If it's streaming, then the body is just the value of the payload.
            body_payload = parameters.get(payload_member, b"")
            body_payload = self._encode_payload(body_payload)
            response.data = body_payload
        elif payload_member is not None:
            # If there's a payload member, we serialized that member to the body.