DEV: Improve add_to_serializer include_* options (#21220)

- Move the old '`define_include_method`' arg to a `respect_plugin_enabled` kwarg

- Introduce an `include_condition` kwarg which can be passed a lambda with inclusion logic. Lambda will be run via `instance_exec` in the context of the serializer instance

This is backwards compatible - old-style invocations will trigger a deprecation message

spec/lib/plugin/instance_spec.rb

@@ -75,6 +75,14 @@ RSpec.describe Plugin::Instance do
 
         # Serializer
         @plugin.add_to_serializer(:trout, :scales) { 1024 }
+        @plugin.add_to_serializer(:trout, :unconditional_scales, respect_plugin_enabled: false) do
+          2048
+        end
+        @plugin.add_to_serializer(
+          :trout,
+          :conditional_scales,
+          include_condition: -> { !!object.data&.[](:has_scales) },
+        ) { 4096 }
 
         @serializer = TroutSerializer.new(@trout)
         @child_serializer = TroutJuniorSerializer.new(@trout)
@@ -100,6 +108,7 @@ RSpec.describe Plugin::Instance do
         expect(@hello_count).to eq(1)
         expect(@serializer.scales).to eq(1024)
         expect(@serializer.include_scales?).to eq(false)
+        expect(@serializer.include_unconditional_scales?).to eq(true)
         expect(@serializer.name).to eq("a trout")
 
         expect(@child_serializer.scales).to eq(1024)
@@ -107,6 +116,25 @@ RSpec.describe Plugin::Instance do
         expect(@child_serializer.name).to eq("a trout jr")
       end
 
+      it "can control the include_* implementation" do
+        @plugin.enabled = true
+
+        expect(@serializer.scales).to eq(1024)
+        expect(@serializer.include_scales?).to eq(true)
+
+        expect(@serializer.unconditional_scales).to eq(2048)
+        expect(@serializer.include_unconditional_scales?).to eq(true)
+
+        expect(@serializer.include_conditional_scales?).to eq(false)
+        @trout.data = { has_scales: true }
+        expect(@serializer.include_conditional_scales?).to eq(true)
+
+        @plugin.enabled = false
+        expect(@serializer.include_scales?).to eq(false)
+        expect(@serializer.include_unconditional_scales?).to eq(true)
+        expect(@serializer.include_conditional_scales?).to eq(false)
+      end
+
       it "only returns HTML if enabled" do
         ctx = Trout.new
         ctx.data = "hello"