Configurable primary update method for CloudNativePG clusters

The chart now templates postgresql.cnpg.primaryUpdateMethod for the CloudNativePG clusters it manages, defaulting to switchover. Operators can set it to restart in their values so the primary pod is recreated in place instead of being handed off to a replica, which is useful when a roll changes the archive path (for example, moving from in-tree barmanObjectStore to the Barman Cloud Plugin) and the old primary cannot archive until its sidecar is present.