Handle the "unreadable" gceZone in AlloyDBInstance

[maqiuyujoyce]

Checklist

• I did not find a related open enhancement request.
• I understand that enhancement requests filed in the GitHub repository are by default low priority.
• If this request is time-sensitive, I have submitted a corresponding issue with GCP support.

Describe the feature or resource

gceZone is never returned when getting an AlloyDBInstance even if it was configured explicitly. So this field is basically unreadable right now. We should handle it properly.

(The TF resource also has the same issue: hashicorp/terraform-provider-google#13378)

Additional information

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Importance

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