DEV: First pass of ThemeSettingsObjectValidator (#25624)

Why this change?

This is a first pass at adding an objects validator which main's job is
to validate an object against a defined schema which we will support. In
this pass, we are simply validating that properties that has been marked
as required are present in the object.

spec/lib/theme_settings_object_validator_spec.rb

@@ -0,0 +1,72 @@
+# frozen_string_literal: true
+
+RSpec.describe ThemeSettingsObjectValidator do
+  describe "#validate" do
+    it "should return the right array of error messages when properties are required but missing" do
+      schema = {
+        name: "section",
+        properties: {
+          title: {
+            type: "string",
+            required: true,
+          },
+          description: {
+            type: "string",
+            required: true,
+          },
+          links: {
+            type: "objects",
+            schema: {
+              name: "link",
+              properties: {
+                name: {
+                  type: "string",
+                  required: true,
+                },
+                child_links: {
+                  type: "objects",
+                  schema: {
+                    name: "child_link",
+                    properties: {
+                      title: {
+                        type: "string",
+                        required: true,
+                      },
+                      not_required: {
+                        type: "string",
+                      },
+                    },
+                  },
+                },
+              },
+            },
+          },
+        },
+      }
+
+      errors = described_class.new(schema:, object: {}).validate
+
+      expect(errors).to eq(title: ["must be present"], description: ["must be present"])
+
+      errors =
+        described_class.new(
+          schema: schema,
+          object: {
+            links: [{ child_links: [{}, {}] }, {}],
+          },
+        ).validate
+
+      expect(errors).to eq(
+        title: ["must be present"],
+        description: ["must be present"],
+        links: [
+          {
+            name: ["must be present"],
+            child_links: [{ title: ["must be present"] }, { title: ["must be present"] }],
+          },
+          { name: ["must be present"] },
+        ],
+      )
+    end
+  end
+end