Support migration from ChirpStack V4

[happyRip]

Summary

Support exporting devices from https://www.chirpstack.io/docs/

Why do we need this?

New version of ChirpStack was released. v4 breaking changes

What is already there? What do you see now?

We support migration from ChirpStack V3.

What is missing? What do you want to see?

Support for device migration from ChirpStack V4.

Environment

v0.9.0-rc0

How do you propose to implement this?

Use gRPC API or REST API to export devices.

How do you propose to test this?

Test locally with chirpstack/chirpstack-docker which works out of the box now (If running V3 needs adding execute permissions to database init files since it was fixed after switching to V4).

Can you do this yourself and submit a Pull Request?

I can do this myself.