feat: Implement View Helper pattern (#3278)

* #1263 - creating new module for view-helper

* #1263 - creating Product and ProductViewModel

* #1263 - creating interfaces for View and ViewHelper

* #1263 - creating ProductViewHelper and its test

* #1263 - creating Console View for Product

* #1263 - creating ProductController

* #1263 - creating main App and its test

* #1263 - adding license header

* #1263 - keeping module in alphabetical order

* #1263 - creating README and diagrams

* #1263 apply spotless

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Co-authored-by: Ilkka Seppälä <iluwatar@users.noreply.github.com>

view-helper/etc/view-helper-sequence-diagram.puml

@@ -0,0 +1,15 @@
+@startuml
+actor Client
+participant Controller
+participant ViewHelper
+participant View
+participant Product
+participant ProductViewModel
+
+Client -> Controller : handle(product)
+Controller -> ViewHelper : prepare(product)
+ViewHelper -> Product : access data
+ViewHelper -> ProductViewModel : return formatted view model
+Controller -> View : render(viewModel)
+View -> Console : display output
+@enduml

view-helper/etc/view-helper.puml

@@ -0,0 +1,45 @@
+@startuml
+package com.iluwatar.viewhelper {
+    interface View<T> {
+        +render(T model)
+    }
+
+    interface ViewHelper<S, T> {
+        +prepare(S source): T
+    }
+
+    class Product {
+        -name: String
+        -price: BigDecimal
+        -releaseDate: LocalDate
+        -discounted: boolean
+    }
+
+    class ProductViewModel {
+        -name: String
+        -price: String
+        -releasedDate: String
+    }
+
+    class ProductViewHelper {
+        +prepare(Product): ProductViewModel
+    }
+
+    class ConsoleProductView {
+        +render(ProductViewModel)
+    }
+
+    class ProductController {
+        -helper: ViewHelper<Product, ProductViewModel>
+        -view: View<ProductViewModel>
+        +handle(Product)
+    }
+}
+Product --> ProductViewHelper
+ProductViewHelper ..|> ViewHelper
+ConsoleProductView ..|> View
+ProductViewHelper --> ProductViewModel
+ProductController --> ProductViewHelper
+ProductController --> ConsoleProductView
+
+@enduml