Explicitly add the dwell time duration

[adriansmares]

Summary

References https://github.com/TheThingsNetwork/lorawan-frequency-plans/pull/48

Changes

• Explicitly add the dwell time duration to the dwell time configuration while enabling the dwell time
  • From what regulations I could find about the AS923 countries, the 400ms duration was used everywhere
  • Should anyone stray away from this common value, we will fix it in a frequency plan dedicated to that country

Notes for Reviewers

The lack of duration fails the validation: https://github.com/TheThingsNetwork/lorawan-stack/blob/9d58a370c158a60072ed841370a5e7a97b67f419/pkg/frequencyplans/frequencyplans.go#L429-L431 During my tests I only verified that disabling works, which was not enough.

I will add CI checks as part of https://github.com/TheThingsNetwork/lorawan-frequency-plans/issues/38 which will catch these issues in the future.

Checklist

• [x] Scope: The referenced issue is addressed, there are no unrelated changes.
• [x] Compatibility: The changes are backwards compatible, they don't break existing deployments.
• [x] Testing: The changes are tested.
• [ ] Documentation: Relevant documentation is added or updated.