Export v2 devices in v3 format

[neoaggelos]

Summary

Closes #764 . This is the v2 part of migrating end devices to the v3.

Changes

• Adds ttnctl devices export <device-id> command
• Adds ttnctl devices export-all command

Example

$ ttnctl devices export-all --frequency-plan-id EU_863_870 | ttn-lw-cli dev create --application-id my-v3-app

It is required to pass the frequency plan ID (v2 devices don't have one). We could probably pick a default one from the handler/router, but I think it's better to let the user decide.

Review

@htdvisser please review CLI and usage. Also, running ttnctl locally updates some packages in go.mod. Not sure how to proceed with these. @rvolosatovs please review the MAC settings

[CLAassistant]

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