option to add Update similar to UpdateAny but for specific attachment

[AmitKumarDas]

Problem Statement: Currently GenericController supports updating any attachment via UpdateAny tunable. However, there can be cases when GenericController should support fine granular update policies on a per attachment basis. In such a case, UpdateAny will not be set and Update will be set against a particular attachment kind.

[AmitKumarDas]

Solution: Spec.Attachments[n].UpdateStrategy is by default set to "OnDelete". In other words, update can only proceed if that attachment is first deleted. This property can be made use of to set updates against particular resource kind(s) only even when UpdateAny is set to true. In other words, do not set this property against the attachments if we want to avoid updating that particular attachment. In addition, set other attachment kind(s) to following if we want to update them.

    updateStrategy:
      method: InPlace