added basic sample to reproduce composite error

[shainegordon]

This reproduces the bug described in https://github.com/Finbuckle/Finbuckle.MultiTenant/issues/238

To recreate this, all you need to do is start the application and goto https://localhost:7208/acme/create-client

You'll be present with the standard .NET development exception UI with the error InvalidOperationException: Unable to track an entity of type 'Client' because its primary key property 'TenantId' is null.