Bug: EthersAwsKmsSignerConfig requires credentials but SSO usage omits it

[cuonghx-dev]

Description

EthersAwsKmsSignerConfig in packages/ethers-aws-kms-signer/src/aws-kms-signer.ts defines credentials as required:

export type EthersAwsKmsSignerConfig = {
  credentials: AwsCredentialIdentityProvider | AwsCredentialIdentity;
  region: string;
  keyId: string;
};

However, the "AWS SSO" test constructs a signer without credentials, relying on the SDK's default credential chain. The credentials field should be optional to support SSO and other default credential resolution methods.

Expected

export type EthersAwsKmsSignerConfig = {
  credentials?: AwsCredentialIdentityProvider | AwsCredentialIdentity;
  region: string;
  keyId: string;
};