### 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_
if method == "POST" and path == "/":
            parsed_data = json.loads(to_str(data))
            action = headers.get("X-Amz-Target") or ""
            action = action.split(".")[-1]
            if response.status_code == 501:
                if action == "GetPublicKey":
                    return handle_get_public_key(parsed_data, response)
                if action == "GenerateDataKeyPair":
                    return generate_data_key_pair(parsed_data, response)
                if action == "GenerateDataKeyPairWithoutPlaintext":
                    return generate_data_key_pair_without_plaintext(parsed_data, response)
                if action == "Sign":
                    return sign(parsed_data, response)
            if response.status_code == 400 and action == "DescribeKey":
                return search_key_pair(parsed_data, response)
            if action == "ListKeys":
                add_key_pairs(response)
UPDATE_KMS = ProxyListenerKMS()