Add support for RID specific package dependencies

[glopesdev]

In the path towards cross-platform deployment it will be useful to have a mechanism to process RID-specific assets such as native dependencies. In the past the bootstrapper has supported CoApp for this purpose, which has been permanently decommissioned for a while now.

.NET Core 2.x introduced the concept of RID-specific package dependencies using the runtime.json file, which is also now deprecated (see NuGet/Home#5862). The cross-platform .NET framework itself is now using runtime targeting packs to work around the problem, but no real replacement for runtime.json has emerged, and the need remains to have standards in place for third-party package maintainers.