Provide option to remove/disable Relay's second channel

[StevenCellist]

Summary

I would like to see a (CLI) option to disable or remove the second channel of a relay.

Current Situation

The second channel is disabled by default, and as soon as a Frequency is set, the second channel is activated (with default DR0, wouldn't it be better to set it to the band's default WoR/ACK DR?). However, there is no way to disable this second channel; e.g. Frequency of 0MHz is not allowed (which is how channels are 'removed' in e.g. NewChannelReq).

The only way to 'remove' a second channel is by removing the relay altogether and completely reconfiguring it which is a huge pain in the arse for a somewhat configured relay.

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

A user may wish to remove a relay's second channel. without completely resetting the relay.

Proposed Implementation

E.g. one of:

• Provide a boolean (CLI) switch to enable/disable the second channel
• Allow the Frequency to be set to 0MHz (and have this listed as an option to disable the second channel)
• Possibly a delete request for the second channel, such as for the uplink-forwarding-rules?

Contributing

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

Validation

• [ ] The feature is tested in a staging environment.
• [ ] The feature is documented in The Things Stack Documentation

Code of Conduct

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