implementation of extension methods to configure databus

Implementation of Fluent API (`ConfigureAzureDataBus`) to provide code-based configuration alternative to config section (`AzureDataBusConfig`).
Both utilize `AzureDataBusGuard` to perform validation.

Please review @yvesgoeleven @johnsimons @SimonCropp 
Don't think so. `string.IsNullOrWhiteSpace` will reject empty string and empty string is valid in our case. Casting is unnecessary here `basePath == null`.