added DatabaseService to get list of database types from server

Oqtane.Client/Services/Interfaces/IDatabaseService.cs

@@ -0,0 +1,11 @@
+using Oqtane.Models;
+using System.Collections.Generic;
+using System.Threading.Tasks;
+
+namespace Oqtane.Services
+{
+    public interface IDatabaseService
+    {
+        Task<List<Database>> GetDatabasesAsync();
+    }
+}