Support TX channel configuration in dynamic channel regions

[johanstokking]

Summary

This fixes default and custom RX2 parameters as well as Class C in dynamic channel plan regions.

Changes

• Update The Things Industries gateway protocol API with support for TX channel configuration. This allows for configuring TX frequency and bandwidth if it deviates from the frequency plan's downlink channels
• Configure default RX2 parameters as TX channel to save bandwidth
• Extend GS frontend test bench with a custom frequency and fix Class C parameters

Testing

Steps

1. Register a gateway using The Things Industries gateway protocol
2. Register an EU868 end device with class C enabled
3. Configure the end device with data rate 6 (SF7, 250 KHz) for RX2 and enable class C
4. Let the end device join
5. Send downlink messages
6. Repeat step 3 with standard data rate for RX2 (any SF at 125 KHz)

Results

Downlink messages should arrive.

Checklist

• Scope: The referenced issue is addressed, there are no unrelated changes.
• Compatibility: The changes are backwards compatible with existing API, storage, configuration and CLI, according to the compatibility commitments in README.md for the chosen target branch.
• Documentation: Relevant documentation is added or updated.
• Testing: The steps/process to test this feature are clearly explained including testing for regressions.
• Infrastructure: If infrastructural changes (e.g., new RPC, configuration) are needed, a separate issue is created in the infrastructural repositories.
• Changelog: Significant features, behavior changes, deprecations and fixes are added to CHANGELOG.md.
• Commits: Commit messages follow guidelines in CONTRIBUTING.md, there are no fixup commits left.