Add SAS token function for the service bus/event hub resource

Developers wishing to send messages to the service bus using the SAS token need to create the token themselves, as described here:
https://docs.microsoft.com/en-us/rest/api/eventhub/generate-sas-token

This is error prone and also convoluted to do in bicep because of the hashing step.

Ideally this should be available as a native property on the service bus resource. In the meantime, we should also be able to add support in bicep to have a list*() function that is able to create the SAS token for the service bus resource:
`value: serviceBus.generateSAStoken()`

Start with the linked Microsoft documentation for generating an Event Hubs SAS token and compare the two requested directions: a native service bus property and a Bicep list*() function such as serviceBus.generateSAStoken(). Done means the supported design is settled and the function or property can produce the documented token without requiring a manual hashing step.

Written by the indexing model from the issue text.