dotnet-svcutil --update should not reorder the class definitions

**Describe the bug**
When updating a service reference based off the same exact WSDL, the order of the classes in the `Reference.cs` file changes, which makes it less version control friendly. It also makes it harder to determine what changed when the WSDL did change.

**To Reproduce**
Steps to reproduce the behavior:
1. Create a new service reference from a WSDL
2. Make a copy of the `Reference.cs` file
3. Run `dotnet-svcutil --update` with your service reference
4. Compare the `Reference.cs` files generated in step 2 and 3.

**Expected behavior**
When running `dotnet-svcutil --update` on a WSDL that has not change, the order of the classes in `Reference.cs` should stay the same.

**Screenshots**
N/A

**Additional context**
using dotnet-svcutil 2.0.1