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/GreenField/StoresController.cs

@@ -2,6 +2,7 @@ using System;
 using System.Collections.Generic;
 using System.Linq;
 using System.Threading.Tasks;
+using BTCPayServer.Abstractions.Constants;
 using BTCPayServer.Client;
 using BTCPayServer.Client.Models;
 using BTCPayServer.Data;