Support custom data rate on LoRa standard channel for fixed channel band plans

[virtualguy]

Summary

On fixed channel band plans (AU915/US915) the LoRa standard channels are fixed at SF8 / 500 kHz. However devices can be encouraged to use faster data rates (SF7 / 500 kHz) by sending NewChannelReq with different parameters. This allows better of the LoRa standard channels with less airtime

Current Situation

NS ignores the min/max data rate from the frequency plan

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

Allows network operators to make better use of spectrum by overriding the fixed data rate

Proposed Implementation

Allow override from the frequency plan

Contributing

• [X] I can help by doing more research.
• [X] 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.

Code of Conduct

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