Emit Packet Error Rate as part of the uplink messages

[adriansmares]

Summary

References https://github.com/TheThingsIndustries/lorawan-stack/issues/4205

Changes

• Add the packet error rate field to the API.
• Fill in the packet error rate field while processing uplinks.

Testing

N/A. This is just a new API field which existing Network Server functionality which is already tested.

Regressions

N/A.

Notes for Reviewers

Writing the implementation explanation was longer than the actual 5 line implementation. Hence the PR.

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.