fix(storage iam policies): Make description field optional in Condition struct

[nhudson]

When requesting IAM policies with conditions (options_requested_policy_version version 3) using the get_iam_policy function, a de-serialization error occurs if the condition doesn't include a description. This is because the description field in the Condition struct is currently defined as a required String.

This PR modifies the Condition struct to make the description field optional by changing its type to Option<String> and adding the #[serde(default)] attribute. This allows the struct to be correctly de-serialized even when the description is missing from the API response.

[yoshidan]

Thanks!