Produce NuGet packages on release build

[bart-vmware]

This enables to detect packaging issues early in pull requests, without increasing build times during debug.

• Because the default for IsPackable varies per SDK, we should:
  • Explicitly set <IsPackable>true</IsPackable> in every project we'd like to produce a NuGet package for
  • Explicitly set <IsPackable>false</IsPackable> in every other project (except tests), such as utilities, benchmarks, examples, etc.
• Move packaging-related settings from sharedproject.props into new file sharedpackage.props
  • In sharedtest.props:
    • Set <IsPackable>false</IsPackable>
  • In shared.props:
    • Import sharedpackage.props only when IsPackable is true
  • In sharedpackage.props:
    • Set <GeneratePackageOnBuild>True</GeneratePackageOnBuild> for release builds
    • Consider to activate EnablePackageValidation for extra checks.
• Update azure-pipelines.yml to take above changes into account