users: convenience functions for converting from BP

The types currently map directly, so it's simple to convert from one to
the other.

internal/users/users.go

@@ -1,5 +1,7 @@
 package users
 
+import "github.com/osbuild/osbuild-composer/internal/blueprint"
+
 type User struct {
 	Name        string
 	Description *string
@@ -16,3 +18,23 @@ type Group struct {
 	Name string
 	GID  *int
 }
+
+func UsersFromBP(userCustomizations []blueprint.UserCustomization) []User {
+	users := make([]User, len(userCustomizations))
+	for idx := range userCustomizations {
+		// currently, they have the same structure, so we convert directly
+		// this will fail to compile as soon as one of the two changes
+		users[idx] = User(userCustomizations[idx])
+	}
+	return users
+}
+
+func GroupsFromBP(groupCustomizations []blueprint.GroupCustomization) []Group {
+	groups := make([]Group, len(groupCustomizations))
+	for idx := range groupCustomizations {
+		// currently, they have the same structure, so we convert directly
+		// this will fail to compile as soon as one of the two changes
+		groups[idx] = Group(groupCustomizations[idx])
+	}
+	return groups
+}