Explicitly add the dwell time duration

[adriansmares]

Summary

References #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 #38 which will catch these issues in the future.

Checklist

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