Update packaging (probably release as strong-name only)

[reisenberger]

In line with App-vNext/Polly#442

EDIT: Now addressed by #19, #21, #22

[af4jm]

Microsoft.Extensions.Http.Polly references Polly 6.0.1 which is signed while this package references Polly 5.9 which is unsigned, making it impossible for a project to reference both NuGet packages.

[reisenberger]

We are hoping to release the updated package in the next few days. Pending that, you can build against a version of the new package immediately by running the build locally from the v20dev branch. This will create a .nupkg in the \artifacts\nuget-package folder which is expected to be identical to the released package, except in naming/numbering.

[reisenberger]

Closed by #22 . The new package is released on nuget