Add optional argument for max delivery attempts when creating or updating subscription with dead lettering

[danavaziri-ga]

To make the sample python code resemble the UI and CPS behavior a bit more we could add an optional max_delivery_attempts argument instead of setting the value to 10(when we create subscription) or 20(when we update subscription). If the user decides not to set the max_delivery_attempts argument we can set it to the default of 5 just like CPS does.