Expose BaseEntityType property via Configuration to set it from outside

[ChristianPardun]

It would be very helpful to have the possibility to set the BaseEntityType property (ODataT4CodeGenerator.ODataClientTemplate.BaseEntityType) via configuration from outside. (Like setting a custom namespace)

Then, in some scenarios, it might be possible to extend/inherit or set a customer BaseEntityType, which finally inherits from Microsoft.OData.Client.BaseEntityType class.