[PM-28555] Add idempotent sproc to create My Items collections (#6801)

* Add sproc to create multiple default collections. 
  SqlBulkCopy implementation is overkill for most cases.
  This provides a lighter weight sproc implementation for smaller
  data sets.
* DRY up collection arrangement
* DRY up tests because bulk and non-bulk share same behavior
* use EF native AddRange instead of bulk insert, because
  we expect smaller data sizes on self-host

src/Infrastructure.EntityFramework/Repositories/DatabaseContext.cs

@@ -17,8 +17,6 @@ using Microsoft.EntityFrameworkCore.Storage.ValueConversion;
 using DP = Microsoft.AspNetCore.DataProtection;
 
 
-#nullable enable
-
 namespace Bit.Infrastructure.EntityFramework.Repositories;
 
 public class DatabaseContext : DbContext