Breaking: Custom Domains & Instances Get Read-Only Fields; New Export Timestamp for ESA

Developers, be aware of two breaking changes: the state property within the CustomDomain schema and the maintenanceSchedule property within the Instance schema have both been made read-only. Any existing code attempting to modify these fields will now fail and must be updated to reflect their immutable status. On the non-breaking front, the ExportMetadata schema now includes a new exportTime property. This field, typed as a string, provides the Unix epoch in microseconds when an export was created, specifically to aid in identifying cloned source Elite System Activity (ESA) BigQuery datasets for O2C direct dataset migrations. This enhances the migration capabilities previously introduced for ESA.