### 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_
"GenerateDataKeyWithoutPlaintext",
    "ReEncryptFrom",
    "ReEncryptTo",
    "Sign",
    "Verify",
    "GetPublicKey",
    "CreateGrant",
    "RetireGrant",
    "DescribeKey",
    "GenerateDataKeyPair",
    "GenerateDataKeyPairWithoutPlaintext",
]
KEY_ID = "KeyId"
GRANTEE_PRINCIPAL = "GranteePrincipal"
RETIRING_PRINCIPAL = "RetiringPrincipal"
OPERATIONS = "Operations"
GRANT_ID = "GrantId"
GRANT_TOKENS = "GrantTokens"
NAME = "Name"
CONSTRAINTS = "Constraints"
ISSUING_ACCOUNT = "IssuingAccount"
CREATION_DATE = "CreationDate"
class KMSBackend(RegionBackend):
    
    grants: Dict[str, Dict]
    
    markers: Dict[str, List]
    
    key_pairs: Dict
    def __init__(self):