Replace CoApp package overlay with runtime.json for native dependencies

[glopesdev]

Until now, OpenCV.NET relied on the legacy CoApp powershell tools to generate package overlays to be installed on demand depending on build configuration. This turned out to be a brittle solution in the long-run, as CoApp is no longer maintained, and the community has moved on into new cross-platform dependency deployment for .NET 5.0 and .NET 6.0.

One of the most promising approaches is the one followed by the ClangSharp project, which co-opts the runtime.json file which is used by nuget and the build system to deploy platform-specific assets in .NET Core and beyond. This approach is particularly exciting as it is also backwards compatible with .NET framework. Although it is an undocumented feature with some drawbacks, it does provide a seamless experiencing of downloading packages on demand and copying them to the application output file.

More information can be found in NuGet/Home#10571 and in various other issues raised in dotnet/runtime and dotnet/sdk raised recently.

Starting with next version of OpenCV.NET, we want to leverage this approach to manage native dependency distribution.

[glopesdev]

Fixed in ea9b439