Allow loading extensions from Nuget

Leveraging existing Nuget loading infrastructure so that new extensions can be loaded via a nuget feed (instead of copying DLLs over)

Start by locating the existing NuGet loading infrastructure and the extension-loading entry points in the Azure Functions host. Determine how a NuGet feed should supply extensions instead of copied DLLs, then verify that extensions can be discovered and loaded through that path.

Written by the indexing model from the issue text.