Refactor Network Server end device registry Set method validation

[adriansmares]

Summary

References https://github.com/TheThingsIndustries/lorawan-stack/pull/3850#issuecomment-1789261038

We should refactor the Network Server NsEndDeviceRegistry.Set validation logic in order to make it easier to maintain and review.

Current Situation

The Set RPC works using field mask paths towards two end devices - the end device in the update request, and the end device which is stored. This makes the logic hard to validate and asses, and adds a lot of complexity to an RPC which is not called very often.

Why do we need this? Who uses it, and when?

In order to be able to actually check the correctness of the RPC, and ensure that we really correctly validate requests.

Proposed Implementation

1. Merge the end device patch to the stored (complete) stored end device.
2. Recursively validate the end device object as a whole.

Contributing

• I can help by doing more research.
• I can help by implementing the feature after the proposal above is approved.
• I can help by testing the feature before it's released.

Code of Conduct

• I agree to follow TTN's Community Code of Conduct.