DEV: Refactor subclasses in ThemeSettingsManager to individual files (#25605)

Why this change?

One Ruby class per file improves readability

spec/lib/theme_settings_manager/objects_spec.rb

@@ -0,0 +1,37 @@
+# frozen_string_literal: true
+
+RSpec.describe ThemeSettingsManager::Objects do
+  fab!(:theme)
+
+  before { SiteSetting.experimental_objects_type_for_theme_settings = true }
+
+  it "can store a list of objects" do
+    objects_setting =
+      described_class.new(
+        "some_objects_setting",
+        [{ "title" => "Some title", "description" => "Some description" }],
+        theme,
+        schema: {
+          name: "Some Object",
+          fields: {
+            title: {
+              type: "string",
+            },
+            description: {
+              type: "string",
+            },
+          },
+        },
+      )
+
+    expect(objects_setting.value).to eq(
+      [{ "title" => "Some title", "description" => "Some description" }],
+    )
+
+    objects_setting.value = [{ title: "title 1", description: "description 1" }]
+
+    expect(objects_setting.value).to eq(
+      [{ "title" => "title 1", "description" => "description 1" }],
+    )
+  end
+end