Improve handling of NamedSignalValue

[erikbosch]

Fixes #30

I used VSS 4.2 as basis for testing, so I added that as well to our example collection. See end of 4.2 *.vspec file to find added testcases.

How to test:

Start Databroker with 4.2 (could be std 4.2)

erik@debian6:~/kuksa-databroker$ cargo run --bin databroker -- --metadata ~/kuksa-can-provider/mapping/vss_4.2/vss_dbc.json

Run dbcfeeder

./dbcfeeder.py

Use kuksa-client to test results.

Test Client> getValue Vehicle.Cabin.Infotainment.HMI.SpeedUnit
{
    "path": "Vehicle.Cabin.Infotainment.HMI.SpeedUnit",
    "value": {
        "value": "KILOMETERS_PER_HOUR",
        "timestamp": "2024-09-20T10:25:27.978556+00:00"
    }
}

Test Client> getValue Vehicle.Cabin.Infotainment.Media.Played.Album
{
    "path": "Vehicle.Cabin.Infotainment.Media.Played.Album",
    "value": {
        "value": "DI_SPEED_KPH",
        "timestamp": "2024-09-20T10:25:50.193728+00:00"
    }
}

Test Client> getValue Vehicle.Cabin.Infotainment.Media.Volume
{
    "path": "Vehicle.Cabin.Infotainment.Media.Volume",
    "value": {
        "value": 1,
        "timestamp": "2024-09-20T10:25:55.723813+00:00"
    }
}