Include the license identifier in the NuGet package

[reduckted]

Is your feature request related to a problem? Please describe.
For automated tools that gather the license of NuGet packages, it's really handy for the license to be specified in the NuGet package (specifically in the .nuspec file).

Describe the solution you'd like
Include the license identifier in the NuGet package. This is all you need to add to the project file (same as Autofac):

<PropertyGroup>
    <PackageLicenseExpression>MIT</PackageLicenseExpression>
</PropertyGroup>

Describe alternatives you've considered
N/A

Additional context
Some links to documentation:

• https://docs.microsoft.com/en-us/nuget/reference/nuspec#license
• Self-Contained NuGet Packages - License NuGet/Home#4628
• 'licenseUrl' as package metadata is now deprecated, use 'license' instead NuGet/Announcements#32
• 'licenseUrl' as package metadata is now deprecated, use 'license' instead NuGet/Announcements#32