Add support for extra_fields query parameter

Api now supports optional fields.
These are included with the extra_fields query param.

Syntax: extra_fields[type]=field1,field2

spec/controllers/concerns/extra_fields_spec.rb

@@ -0,0 +1,60 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe ExtraFields do
+  let(:dummy_controller) { Api::V1::BaseController.new.extend ExtraFields }
+
+  describe "#invalid_query_param" do
+    it "renders error" do
+      allow(dummy_controller).to receive(:render) {}
+      dummy_controller.invalid_query_param("param", :unprocessable_entity, "error message")
+      expect(dummy_controller).to have_received(:render).with(
+        json:
+          {
+            errors:
+            [{
+              code: 422,
+              detail: "error message",
+              source: { parameter: "param" },
+              status: :unprocessable_entity,
+              title: "Invalid query parameter"
+            }]
+          },
+        status: :unprocessable_entity
+      )
+    end
+  end
+
+  describe "#extra_fields" do
+    context "when fields present and available" do
+      it "returns extra fields" do
+        allow(dummy_controller).to receive(:params).
+          and_return({ extra_fields: { customer: "balance" } })
+        expect(dummy_controller.extra_fields(:customer, [:balance])).to eq([:balance])
+      end
+    end
+
+    context "when fields missing" do
+      it "returns empty arr" do
+        allow(dummy_controller).to receive(:params).and_return({})
+        expect(dummy_controller.extra_fields(:customer, [:balance])).to eq([])
+      end
+    end
+
+    context "when fields not in available fields" do
+      it "calls invalid_query_param" do
+        allow(dummy_controller).to receive(:invalid_query_param) {}
+        allow(dummy_controller).to receive(:params).
+          and_return({ extra_fields: { customer: "unknown" } })
+        dummy_controller.extra_fields(:customer, [:balance])
+
+        expect(dummy_controller).to have_received(:invalid_query_param).with(
+          "extra_fields[customer]",
+          :unprocessable_entity,
+          "Unsupported fields: unknown"
+        )
+      end
+    end
+  end
+end