Add simple feature flag system (#34038)

Co-authored-by: Claire <claire.github-309c@sitedethib.com>

spec/lib/mastodon/feature_spec.rb

@@ -0,0 +1,30 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe Mastodon::Feature do
+  around do |example|
+    original_value = Rails.configuration.x.mastodon.experimental_features
+    Rails.configuration.x.mastodon.experimental_features = 'fasp,fetch_all_replies'
+    example.run
+    Rails.configuration.x.mastodon.experimental_features = original_value
+  end
+
+  describe '::fasp_enabled?' do
+    subject { described_class.fasp_enabled? }
+
+    it { is_expected.to be true }
+  end
+
+  describe '::fetch_all_replies_enabled?' do
+    subject { described_class.fetch_all_replies_enabled? }
+
+    it { is_expected.to be true }
+  end
+
+  describe '::unspecified_feature_enabled?' do
+    subject { described_class.unspecified_feature_enabled? }
+
+    it { is_expected.to be false }
+  end
+end