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.Plugins.Test/Data/TestPluginData.cs

@@ -0,0 +1,14 @@
+using System;
+using System.ComponentModel.DataAnnotations.Schema;
+
+namespace BTCPayServer.Plugins.Test.Data
+{
+    public class TestPluginData
+    {
+        [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
+        public string Id { get; set; }
+        public DateTimeOffset Timestamp { get; set; }
+        
+        
+    }
+}