Define the outbound authentication abstraction in alknet_core::credentials:
- CredentialProvider trait with get_credentials and refresh_credentials
- CredentialSet enum with ApiKey, Basic, Bearer, S3AccessKey, OidcToken, Custom variants
- ConfigCredentialProvider reads credentials from DynamicConfig.credentials
- SecretStoreCredentialProvider stub returns None for all lookups (Phase 3)
- Wire CredentialProvider into OperationEnv via credentials() method
- Add credentials HashMap field to DynamicConfig
glm-5.1
f8b4fb66b3