Support `measurement.unit`

### Description

https://develop.sentry.dev/sdk/event-payloads/transaction/

```
{
  "measurements": {
    "my.custom.metric": { "value": 456, "unit": "millisecond" }
  }
}
```

`MeasurementValue` class `serialize` and `deserialize` should add support for it, it's currently losing its unit from Hybrid SDKs