[6.0] Codegeneration improvements

High level goals for code generation

[x] Works for server projects which targets .net6 (maybe even for netstandard2.0)
- also works for .Net Framework (what is supported today)
- Keeps support for clients running .net framework, netstandard2.0 and net core (net5.0+) (#274 )
[x] Works cross platform with "dotnet build" Fixed by (#414)
[x] Support portable pdb (#185)

A working sample can be found in Samples repo

Codegen remaining work

[ ] Add test similar to CreateRiaClientFiles_ASPNET_AppDomain for net6 link
- We want a test which verifies that Program.SetupAppConfig in codegeneration program works as expected
[ ] Add dependencies from Tools and Tools.TextTemplate in T4.nuspec link
[ ] Create ServerClassLibNet6 by copying ServerClassLib link
[x] Fix insecurity with Path.GetTempFile()
[ ] Implement or take action on server codegen validation
[x] Support nullable reference types (it generates attributes on client which gives Compile errors).
- see nullable attributes metadata which needs to be "reversed/decompiled" to corresponding c#/vb code
- a good temporary solution could be to ignore properties in code gen
[x] Support "required" properties (it generates an attribute on client which gives Compile errors)
[ ] Support other new property features (if any) that are implemented as attributes that are not allowed to be specified in code ("init" properties?)

New codegen exe todo's:

Program.cs

[ ] // TODO: Remove dependency on MSBuild and then remove any PackageReferences to MSBuild and MSBuildLocator
- This will require splitting "OpenRiaServices.Tools" into 2 separate projects, one with MSbuild tasks and one without (just code generation)

SharedCodeServiceParametersBinder.cs

[ ] Change parameter names to match "default/recommended" names such as "--shared-source-files" This might make the binders redundant and maybe allows some further cleanup of the program // TODO:
[ ] Change name of project (and exe) , from "Tools.CodeGen" to something else such as "Tools.Console"

OpenRIAServices / OpenRiaServices