Re-introduce .NET Standard 2.0 DLLs

[cheenamalhotra]

Addresses #3316

This update brings back .NET Standard DLLs for the lib and ref folders - following the pattern from System.Data.SqlClient NuGet package.

The new NuGet package folder structure shall look like:

lib\
    net462\
    net8.0\
    net9.0\
    netstandard2.0\
ref\
    net462\
    net8.0\
    net9.0\
    netstandard2.0\
runtimes\
    unix\
        lib\
            net8.0\
            net9.0\
    win\
        lib\
            net462\
            net8.0\
            net9.0\

[Copilot]

Pull Request Overview

This PR re-introduces .NET Standard 2.0 DLLs for both the lib and ref folders, aligning with the pattern used by the System.Data.SqlClient NuGet package.

• Updated the nuspec file to add a new targetFramework group for netstandard2.0 and adjust dependency versions.
• Modified the ref project file to include netstandard2.0 and configure proper output paths using the BuildForLib flag.
• Enhanced the build.proj file to include a BuildNetStandard target for building the .NET Standard DLLs.

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
tools/specs/Microsoft.Data.SqlClient.nuspec	Added a new targetFramework group for netstandard2.0 and updated dependencies accordingly.
src/Microsoft.Data.SqlClient/netcore/ref/Microsoft.Data.SqlClient.csproj	Expanded TargetFrameworks to include netstandard2.0 and introduced conditions for BuildForLib output paths.
src/Microsoft.Data.SqlClient/netcore/ref/Microsoft.Data.SqlClient.cs	Wrapped some API members with conditional compilation (#if NET) for targeted builds.
build.proj	Added a BuildNetStandard target and updated dependencies to incorporate .NET Standard builds.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.97%. Comparing base (14bee54) to head (d3999a5).
Report is 25 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3381      +/-   ##
==========================================
+ Coverage   65.07%   67.97%   +2.89%     
==========================================
  Files         298      298              
  Lines       65515    65525      +10     
==========================================
+ Hits        42634    44538    +1904     
+ Misses      22881    20987    -1894     

Flag	Coverage Δ
addons	92.58% <ø> (ø)
netcore	72.34% <ø> (+4.00%)	⬆️
netfx	66.46% <ø> (+0.29%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[benrr101]

@cheenamalhotra Will we be reintroducing tests for netstandard builds? I'm concerned that since we removed a lot of code that was specific to netstandard implementation (at least in the interop space), allowing people to compile against ref binaries for netstandard will result in runtime errors. Just checking that this is something we considered.

[cheenamalhotra]

No, because we are only adding compile time assemblies for .NET Standard, not runtime assemblies, we don't need to add changes to src projects and therefore only ref projects are built for .NET Standard target.

Also, .NET Standard is not a runtime framework, it's only a target framework for libraries and reusable components that are shipped as NuGet packages. When a "runtime" framework targets them, respective runtime assemblies are fetched from dependency packages and resolved by .NET/MsBuild SDK.

This is the model of how SDS NuGet package is shipped, we're using the same for MDS now.

[David-Engel]

LGTM

Where should we document that the NetStandard DLL isn't supported at runtime?