The svcutil tool fails when referencing an IXmlSerializable type that uses the XmlSchemaProviderAttribute

**Describe the bug**
When referencing an assembly containing an `IXmlSerializable` type that needs to match a WSDL declared type in a custom namespace, use of the standard `System.Xml.Serialization.XmlSchemaProviderAttribute` doesn't enable the `dotnet-svcutil` tool to correctly match the type in the referenced assembly. Instead, a new type is generated in the output code.

This prevents us from having a single implementation of types that are used across several web-services.

**Cause**
I've spent time debugging and found that the cause is in the `SchemaExporter.cs` file inside the `InvokeSchemaProviderMethod` where the implementation is searching for the `XmlSchemaProviderAttribute` type contained in the `FrameworkFork`. That is, it's searching for `Microsoft.Xml.Serialization.XmlSchemaProviderAttribute` rather than `System.Xml.Serialization.XmlSchemaProviderAttribute`.

This is similar to the bug that @imcarolwang fixed in #5746 regarding the `IXmlSerializable` interface. The fix is slightly more involved than the one in #5746 as additional types within the `InvokeSchemaProviderMethod` also need to be swapped to refer to the standard framework types rather than the FrameworkFork types.

I'm happy to provide a PR with my proposed fix.

**Expected behavior**
The `dotnet-svcutil` tool would find and use the `System.Xml.Serialization.XmlSchemaProviderAttribute` to resolve the custom namespace declaration and match the type in the referenced assembly to the WSDL type.