Add signing for Nuget packages

Currently, we're not signing [our nuget packages](https://www.nuget.org/profiles/altinn).

*Signed packages allows for content integrity verification checks which provides protection against content tampering. The package signature also serves as the single source of truth about the actual origin of the package and bolsters package authenticity for the consumer.*

https://docs.microsoft.com/nuget/create-packages/sign-a-package

Start with the repository's NuGet package publishing setup and the linked NuGet package-signing documentation. Done means the packages published from the Altinn NuGet profile are signed and consumers can verify their integrity and origin.

Written by the indexing model from the issue text.