FEATURE: modifier API for plugins (#20887)

Introduces a new API for plugin data modification without class-based extension overhead.

This commit introduces a new API that allows plugins to modify data in cases where they return different data rather than additional data, as is common with filtered_registers in DiscoursePluginRegistry. This API removes the need for defining class-based extension points.

When a plugin registers a modifier, it will automatically be called if the plugin is enabled. The core will then modify the parameter sent to it using the block registered by the plugin:
 
```ruby
DiscoursePluginRegistry.register_modifier(plugin_instance, :magic_sum_modifier) { |a, b| a + b }
sum = DiscoursePluginRegistry.apply_modifier(:magic_sum_filter, 1, 2)
expect(sum).to eq(3)
```

Key features of these modifiers:

- Operate in a stack (first registered, first called)
- Automatically disabled when the plugin is disabled
- Pass the cumulative result of all block invocations to the caller

spec/lib/discourse_plugin_registry_spec.rb

@@ -258,4 +258,55 @@ RSpec.describe DiscoursePluginRegistry do
       )
     end
   end
+
+  context "with filters" do
+    after { DiscoursePluginRegistry.clear_modifiers! }
+
+    class TestFilterPlugInstance < Plugin::Instance
+      def enabled?
+        !@disabled
+      end
+
+      def enabled=(value)
+        @disabled = !value
+      end
+    end
+
+    let(:plugin_instance) { TestFilterPlugInstance.new }
+    let(:plugin_instance2) { TestFilterPlugInstance.new }
+
+    it "handles modifiers with multiple parameters" do
+      DiscoursePluginRegistry.register_modifier(plugin_instance, :magic_sum_modifier) do |a, b|
+        a + b
+      end
+
+      sum = DiscoursePluginRegistry.apply_modifier(:magic_sum_modifier, 1, 2)
+      expect(sum).to eq(3)
+    end
+
+    it "handles modifier stacking" do
+      # first in, first called
+      DiscoursePluginRegistry.register_modifier(plugin_instance, :stacking) { |x| x == 1 ? 2 : 1 }
+      DiscoursePluginRegistry.register_modifier(plugin_instance2, :stacking) { |x| x + 1 }
+
+      expect(DiscoursePluginRegistry.apply_modifier(:stacking, 1)).to eq(3)
+    end
+
+    it "handles disabled plugins" do
+      plugin_instance.enabled = false
+      DiscoursePluginRegistry.register_modifier(plugin_instance, :magic_sum_modifier) do |a, b|
+        a + b
+      end
+
+      sum = DiscoursePluginRegistry.apply_modifier(:magic_sum_modifier, 1, 2)
+      expect(sum).to eq(1)
+    end
+
+    it "can handle arity mismatch" do
+      DiscoursePluginRegistry.register_modifier(plugin_instance, :magic_sum_modifier) { 42 }
+
+      sum = DiscoursePluginRegistry.apply_modifier(:magic_sum_modifier, 1, 2)
+      expect(sum).to eq(42)
+    end
+  end
 end

spec/lib/plugin/instance_spec.rb

@@ -827,4 +827,17 @@ RSpec.describe Plugin::Instance do
       expect(callback_called).to eq(false)
     end
   end
+
+  describe "#register_modifier" do
+    let(:plugin) { Plugin::Instance.new }
+
+    after { DiscoursePluginRegistry.clear_modifiers! }
+
+    it "allows modifier registration" do
+      plugin.register_modifier(:magic_sum_modifier) { |a, b| a + b }
+
+      sum = DiscoursePluginRegistry.apply_modifier(:magic_sum_modifier, 1, 2)
+      expect(sum).to eq(3)
+    end
+  end
 end