Add conditional DDI_ADDSERVICES for partial class emitted for registrations

Since the user can turn off this at the static generator level, the incremental one should honor that too.

This means users can turn off `AddServices` being compiled into their API by defining the preprocessor constant/directive `DDI_ADDSERVICES`.

Author: kzu

src/DependencyInjection/IncrementalGenerator.cs

@@ -2,6 +2,7 @@
 using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.Linq;
+using System.Reflection;
 using System.Text;
 using System.Text.RegularExpressions;
 using System.Threading;
@@ -290,6 +291,7 @@ void AddPartial(string methodName, SourceProductionContext ctx, (ImmutableArray<
 
         builder.AppendLine(
           $$"""
+            #if DDI_ADDSERVICES
             using Microsoft.Extensions.DependencyInjection.Extensions;
             using System;
             
@@ -309,6 +311,7 @@ static partial class {{className}}
                     }
                 }
             }
+            #endif
             """);
 
         ctx.AddSource(methodName + ".g", builder.ToString().Replace("\r\n", "\n").Replace("\n", Environment.NewLine));