### 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_
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        for name in shape_members:
            member_shape = shape_members[name]
            ")
            if not location:
                continue
            if name not in parameters:
                # ignores optional keys
                continue
            key = member_shape.serialization.get("name", name)
            value = parameters[name]
            if value is None:
                continue
             == "header":
                response.headers[key] = self._serialize_header_value(member_shape, value)
            el == "headers":
                header_prefix = key
                self._serialize_header_map(header_prefix, response, value)
            el == "statusCode":
                response.status_code = int(value)
    def _serialize_header_map(self, prefix: str, response: HttpResponse, params: dict) -> None:
        """Serializes the header map for the location trait "headers"."""
        for key, val in params.items():
            actual_key = prefix + key
            response.headers[actual_key] = val
    def _serialize_header_value(self, shape: Shape, value: any):