Warn users about empty `frequency_plan_id` in the Console

[bafonins]

Summary

After https://github.com/TheThingsNetwork/lorawan-stack/pull/2785 we allow creating/updating gateways without frequency_plan_id in the Console. While leaving empty frequency_plan_id is allowed we should warn the users about consequences.

Why do we need this?

Make users aware of limitations when creating/updating gateways without frequency_plan_id

What is already there? What do you see now?

The frequency_plan_id field which is not required:

What is missing? What do you want to see?

@KrishnaIyer what are general limitations when creating gateways without frequency_plan_id ?

How do you propose to implement this?

I think the easiest is to show a warning that includes field description and the limitations. Once a frequency plan is selected we can remove the warning and show a regular description.

How do you propose to test this?

Check the frequency_plan_id field w/ and w/o value set.

Can you do this yourself and submit a Pull Request?

yes

[KrishnaIyer]

what are general limitations when creating gateways without frequency_plan_id ?

Basically the GS needs the frequency_plan_id to fetch the band and look up stuff like Data Rate. Without this the GS cannot send downlinks and for some frontends (ex: basic station) cannot receive uplinks/configure the gateway.

A warning like "Without choosing a frequency_plan_id, the packets from the gateway will not be correctly processed" would work I guess.