Don't require Target when doing gNMI requests

[wenovus]

• Auto-populate Target when doing gNMI requests
• Auto-remove Target when responding to gNMI SubscribeRequest that didn't have prefix populated: https://www.openconfig.net/docs/gnmi/gnmi-specification/#2221-path-target

Resolves #263

[DanG100]

Is there a TODO to remove all the places in the lemming that set a target?

[wenovus]

Is there a TODO to remove all the places in the lemming that set a target?

Thanks for catching this. There are indeed a lot of places the code that now unnecessarily take target as an argument or otherwise assume that gNMI requires populating target. I'll create a task for clean-up.