Reverse engineering: Have a way to discover installed providers so you don't always need to specify them in commands

[divega]

Currently the experience with scaffolding is that you have to install the tools and the provider first, and then repeat the name of the provider in the scaffolding command, e.g.:

dotnet ef dbcontext scaffold Name=Chinook Microsoft.EntityFrameworkCore.SqlServer

If we had a way to discover what providers are installed (e.g. via an assembly attribute) we could:

• Remove the requirement to specify the provider if there is only one
• Inform what providers are available if there is more than one
• Compensate for any mismatch between the package names, assembly names, and what users think the name of the provider is (e.g. something like this may help with our SQLite provider if you have installed the .Core package).

cc @bricelam