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/ManageController.APIKeys.cs

@@ -3,6 +3,8 @@ using System.Collections.Generic;
 using System.Globalization;
 using System.Linq;
 using System.Threading.Tasks;
+using BTCPayServer.Abstractions.Extensions;
+using BTCPayServer.Abstractions.Models;
 using BTCPayServer.Client;
 using BTCPayServer.Data;
 using BTCPayServer.Models;