Add column labels for age and lifetime (#419)

Co-authored-by: Christian Georgi <chgeo@users.noreply.github.com>

fiori/app/_i18n/i18n.properties

@@ -12,6 +12,7 @@ DateOfDeath = Date of Death
 PlaceOfBirth = Place of Birth
 PlaceOfDeath = Place of Death
 Age = Age
+Lifetime = Lifetime
 Authors = Authors
 Order = Order
 Orders = Orders

fiori/app/admin-authors/fiori-service.cds

@@ -43,5 +43,10 @@ extend sap.capire.bookshop.Authors with {
   virtual lifetime : String;
 }
 
+annotate AdminService.Authors with {
+  age      @Common.Label : '{i18n>Age}';
+  lifetime @Common.Label : '{i18n>Lifetime}'
+}
+
 // Workaround for Fiori popup for asking user to enter a new UUID on Create
 annotate AdminService.Authors with { ID @Core.Computed; }