feat(step): cache Step#service_result

BACKLOG.md

@@ -220,3 +220,17 @@ Resolved by [b8285e3](/~https://github.com/marian13/convenient_service/commit/b828
   That is not intuitive.
 
 ---
+
+### Consider to replace `CachesReturnValue`
+
+| Priority | Complexity | Status | Tags |
+| - | - | - | - |
+| Medium | Moderate | TODO | performance-optimization, CachesReturnValue |
+
+Services and steps are caching their results utilizing the `CachesReturnValue` plugin.
+
+Although it is very flexible and easy to reuse from the maintenance point of view, that is not the best option from the performance point of view.
+
+In case it starts to cause any visible performance penalty, it can be refactored using regular `||=` or `if defined?`.
+
+---

lib/convenient_service/service/plugins/can_have_steps/entities/step/concern/instance_methods.rb

@@ -207,7 +207,7 @@ def reassignment(name)
                 #   NOTE: `service_result` returns a foreign result that is later converted to own result by `HasResult` middleware.
                 #
                 def service_result
-                  service.klass.result(**input_values)
+                  @service_result ||= service.klass.result(**input_values)
                 end
 
                 ##

spec/lib/convenient_service/service/plugins/can_have_steps/entities/step/concern/instance_methods_spec.rb

@@ -607,6 +607,10 @@ def result
             .with_arguments(**step.input_values)
             .and_return_its_value
         end
+
+        specify do
+          expect { step.service_result }.to cache_its_value
+        end
       end
     end