Add RPC to NS for generating ABP DevAddr

[htdvisser]

Summary:

We should add an RPC to the NS that generates a DevAddr for ABP devices.

Refs #47 (comment), probably also related to #130.

Why do we need this?

Clients such as the CLI and Console need to request a DevAddr from the NS in order to register ABP devices with a properly peering-routable DevAddr (so not one from the experimental range).

What is already there? What do you see now?

The CLI and Console generate DevAddrs for ABP devices from the experimental range. This works fine on private networks, but won't work over peering.

The NS generates DevAddrs from a NetID (or prefix after #130).

What is missing? What do you want to see?

The NS should have functionality to allocate DevAddrs for ABP devices and expose this over an RPC.

How do you propose to implement this?

Basically the same as what it does for OTAA devices, and then exposed as an RPC. Inputs of the generator are ctx, NetID/DevAddrPrefix of the NS, AppID and perhaps (in the future) an approximate location (so that we can later make the DevAddr allocation and matching smarter/faster). It returns a DevAddr.

What can you do yourself and what do you need help with?

I can assist with the API definition and other discussion.

[htdvisser]

@rvolosatovs proposed alternative of generating a DevAddr if unset

[htdvisser]

@johanstokking says that it should be possible to bring your own DevAddr, it should validate with the list of DevAddr prefixes.