Support migration to uint32 RSSI for TTIGW protocol

[johanstokking]

Summary

Internal reference https://github.com/TheThingsIndustries/gateway/issues/32

This supports migrating from float RSSI values to uint32 in protobuf, which are more bandwidth efficient.

Changes

• Support reading both the (now deprecated) float as well as the new uint32 fields

Testing

Steps

1. Connect The Things Indoor Gateway Pro
2. Observe that LoRa channel and signal RSSI in uplink messages
3. Perform a WiFi access point scan

Results

The RSSI values (including the WiFi AP signal strength) should make sense (i.e. not 0).

Regressions

None

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.