Add Author Signature verification for all packages

[andrei-epure-sonarsource]

Inside NuGet config we only verify the signature for Newtonsoft.JSON.

We should verify the signature also for:

• Microsoft.Build 15.9.20 - has signature
• Microsoft.Build.Utilities.Core 15.9.20 - has signature
• System.Text.Json 6.0.6 - has signature
• System.Net.Http 4.3.4 - has signature
• System.ValueTuple 4.5.0 - has signature
• Google.Protobuf 3.25.0 - has signature
• Grpc.Tools 2.59.0 - has signature

because these packages are signed.

How to extract the author signature:

NuGet.exe verify -signatures microsoft.codeanalysis.csharp.workspaces.4.8.0.nupkg

The author signature is the SHA256 :

Signature type: Author
Subject Name: CN=Microsoft Corporation, O=Microsoft Corporation, L=Redmond, S=Washington, C=US
SHA256 hash: 566A31882BE208BE4422F7CFD66ED09F5D4524A5994F50CCC8B05EC0528C1353

Do the same for each of the package and add an entry, moving the author name from the "repository" section to the "author" section.