Plugins: Allow creation of independent DbContexts

This allows plugins to create custom dbcontexts, which would be namespaced in the scheme with a prefix. Migrations are supported too and the table would be prefixed too

BTCPayServer/Controllers/HomeController.cs

@@ -5,6 +5,7 @@ using System.IO;
 using System.Linq;
 using System.Net.Http;
 using System.Threading.Tasks;
+using BTCPayServer.Abstractions.Constants;
 using BTCPayServer.Data;
 using BTCPayServer.HostedServices;
 using BTCPayServer.Models;