Describe the bug
This is part way between a bug and a feature, so feel free to choose which.

Currently, the Microsoft.Azure.ServiceBus client mandates that the namespace endpoint includes a . in it. Therefore ensuring that only "real" domains can be used. This precludes using something that is hosted on localhost. This seems like something that shouldn't be there as locally contacting servicebus should be ok.

Expected behavior
A servicebus namespace can be specified without a . e.g. localhost

Actual behavior (include Exception or Stack Trace)
An exception is thrown on startup.

To Reproduce
Have a namespace that is localhost, start the client and you'll see it fail.

Environment:
All environment, there's nothing specific here.

Notes:
The line of code that does this is:
https://github.com/Azure/azure-sdk-for-net/blob/main/sdk/servicebus/Microsoft.Azure.ServiceBus/src/ServiceBusConnectionStringBuilder.cs#L174

This restriction is not in Azure.Messaging.ServiceBus