Filter custom members in TypeProvider (#4518)

Moves the custom member filtering into TypeProvider so that we can
maintain a clear contract that the Properties, Methods, Constructors
properties only contain providers that we are actually going to
generate.

packages/http-client-csharp/generator/Microsoft.Generator.CSharp/src/CodeModelPlugin.cs

@@ -21,7 +21,7 @@ namespace Microsoft.Generator.CSharp
     [ExportMetadata("PluginName", nameof(CodeModelPlugin))]
     public abstract class CodeModelPlugin
     {
-        private List<LibraryVisitor> _visitors = [new MemberRemoverVisitor()];
+        private List<LibraryVisitor> _visitors = [];
         private static CodeModelPlugin? _instance;
         internal static CodeModelPlugin Instance
         {

packages/http-client-csharp/generator/Microsoft.Generator.CSharp/src/Providers/PropertyProvider.cs

@@ -44,6 +44,8 @@ public class PropertyProvider
 
         internal IEnumerable<AttributeData>? Attributes { get; init; }
 
+        internal PropertyProvider? SpecProperty { get; set; }
+
         // for mocking
 #pragma warning disable CS8618 // Non-nullable field must contain a non-null value when exiting constructor. Consider declaring as nullable.
         protected PropertyProvider()