Move Visual Studio's Utf8JsonStreamReader & extensions to NuGet Shared

[donnie-msft]

Bug

Fixes: NuGet/Home#14579

Description

Moves the Utf8Json helper classes into NuGet Shared.

At this time, I have only moved the implementation used for Visual Studio project.json migration.
The NuGet.ProjectModel implementation references a String resource. However, I can neither add a RESX file to NuGet Shared, nor can I get the compiler to find the Strings in each consuming csproj.
See Draft #6834

Here, Strings is not found:

Adding a Resource file to build\Shared is not supported:

PR Checklist

• Meaningful title, helpful description and a linked NuGet/Home issue
• Added tests
• Link to an issue or pull request to update docs if this PR changes settings, environment variables, new feature, etc.

[nkolev92]

Without moving both implementations, I don't really see the point of moving it to shared.
It just adds work and makes it harder to change things in that file later.

[donnie-msft]

Without moving both implementations, I don't really see the point of moving it to shared. It just adds work and makes it harder to change things in that file later.

Totally! That's why I have linked a Draft PR moving that as well. Feel free to propose ways around the blocking issue I've described in that PR description. Thanks