### 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_
_provider_config: Dict[str, str]
    default_value: str
    override_prefix: str = "PROVIDER_OVERRIDE_"
    def __init__(self, default_value: str):
        self._provider_config = {}
        self.default_value = default_value
    def load_from_environment(self, env: Mapping[str, str] = None):
        if env is None:
            env = os.environ
        for key, value in env.items():
            if key.startswith(self.override_prefix):
                self.set_provider(key[len(self.override_prefix) :].lower().replace("_", "-"), value)
    def get_provider(self, service: str) -> str:
        return self._provider_config.get(service, self.default_value)
    def set_provider_if_not_exists(self, service: str, provider: str) -> None: