Add Network layer BatteryPercentage to ApplicationUplink

[halimi]

Summary

References: #7427

The Network Server already receives the battery percentage via the DevStatus MAC command and stores it. We can forward this value to the Application Server in the ApplicationUplink message.

Changes

• Extend the ApplicationUplink message with the LastBatteryPercentage message
• Add the stored values during the uplink match

Testing

Steps

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Results

Regressions

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Notes for Reviewers

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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.

[johanstokking]

This comment doesn't really match the validation rules. I think the comment needs to be changed to [0..1] interval. That would not be a percentage anymore, which is [0..100].

I hope it is [0, 100] like the comment says, otherwise turn it into a [0, 100] so that it matches the name (percentage), which is what API consumers would find most natural I think.

[halimi]

I've copied this from the end_device.proto and calls it "percentage" but indeed then the validation should be [0, 100]
https://github.com/TheThingsNetwork/lorawan-stack/blob/v3.33/api/ttn/lorawan/v3/end_device.proto#L1142-L1148

  // Latest-known battery percentage of the device.
  // Received via the DevStatus MAC command at last_dev_status_received_at or earlier.
  // Stored in Network Server.
  google.protobuf.FloatValue battery_percentage = 35 [(validate.rules).float = {
    gte: 0,
    lte: 1
  }];

But I'll check it again and change it (or convert it) in my implementation to be a percentage value [0, 100].