add azure.eventhub.namespace.connectionstring.get pkg

[chrisdostert]

User Story:

As a consumer of opspec
I want an azure.eventhub.namespace.connectionstring.get pkg
In order to get a connection string of my azure eventhub namespace

Reference:

• https://github.com/opspec-pkgs/azure.eventhub.namespace.create
• https://github.com/opspec-pkgs/azure.servicebus.namespace.connectionstring.get

[zachpuck]

completed: https://github.com/opspec-pkgs/azure.eventhub.namespace.connectionstring.get