feature: Add gateway pattern (#1297) (#2734)

* Implement Gateway pattern

* Fixed docstrings

* Fixed README.md

* Fixed App

* Fixed pom.xml

* Fixed pom.xml

* Fixed pom.xml

* Fixed pom.xml

* fixed checkstyle errors and directory names

* fixed pom.xml

* fixed checkstyle errors

* Bug fixed

* Bug fixed

* Bug fixed

* Bug fixed and code improvement

* Bug fixed and code improvement

* Bug fixed and update README.md

* update APP.java

* update gateway pattern

* Revert "update gateway pattern"

This reverts commit f31a00c268889b2c6b7690e07847df7247725999.

* update gateway pattern

---------

Co-authored-by: Finn <u7552286@anu.edu.au>

gateway/etc/gateway.urm.puml

@@ -0,0 +1,43 @@
+@startuml GatewayPattern
+package com.iluwatar.gateway{
+class App {
+  +main(args: String[]): void
+}
+
+class GatewayFactory {
+  -gateways: Map<String, Gateway>
+  +registerGateway(key: String, gateway: Gateway): void
+  +getGateway(key: String): Gateway
+}
+
+interface Gateway {
+  {abstract} +execute(): void
+}
+
+class ExternalServiceA {
+  +execute(): void
+}
+
+class ExternalServiceB {
+  +execute(): void
+}
+
+class ExternalServiceC {
+  +execute(): void
+  +error(): void
+}
+
+App --> GatewayFactory : Uses
+
+
+GatewayFactory --> Gateway : Creates
+
+GatewayFactory --> ExternalServiceA : Registers
+GatewayFactory --> ExternalServiceB : Registers
+GatewayFactory --> ExternalServiceC : Registers
+
+ExternalServiceA --> Gateway : Implements
+ExternalServiceB --> Gateway : Implements
+ExternalServiceC --> Gateway : Implements
+
+@enduml