Feature request: Microsoft.DBforPostgreSQL/flexibleServers/maintenances/write control action

- We are an ISV developing a solution (Azure Managed Application) that includes PostgreSQL Flexible server.
- As part of our solution, we want to give our customers the ability to change maintenance windows without altering any other configuration of the PostgreSQL server. This can be achieved by [specifying allowed control actions](https://learn.microsoft.com/en-us/partner-center/marketplace-offers/plan-azure-app-managed-app#customize-allowed-customer-actions-optional) for our customers as part of the Azure Managed Application configuration.
- Unfortunately, PostgresSQL Flexible Server does not expose the Microsoft.DBforPostgreSQL/flexibleServers/maintenances/write control action we need to enable our customers to only make this change.
- Similar control actions are available for MySQL (Microsoft.DBforMySQL/flexibleServers/maintenances/write) and SQL Server (Microsoft.Sql/servers/databases/maintenanceWindows/write).
- Please add this control action to PostgreSQL Flexible Server so we can service our mutual customers in this.