Adding support for KmlLayer without relying on MapEffect

As developer I would like to be able to add different layers (Kml, GeoJson, etc) to the GoogleMap composable.

A KmlLayer needs a GoogleMap instance to be built. `KmlLayer(GoogleMap map, int resourceId, Context context)`
At the moment the only way to add a Kml layer is using the experimental `MapEffect` (which it may get removed in the future).

Loading a KmlLayer is also slow so it needs to be done in a background thread.

Considering all of the above I came up with a solution but it would be nice to have a different way to do it.
This solution will not work anymore if MapEffect is removed.

```
val scope = rememberCoroutineScope()
val context = LocalContext.current
val aKmlLayer: KmlLayer? = null
GoogleMap(
...
) {
MapEffect { gMap ->
scope.launch {
withContext(Dispatchers.IO) {
aKmlLayer = KmlLayer(gMap, R.raw.kml_file, context)
withContext(Dispatchers.Main) {
aKmlLayer?.addLayerToMap()
}
}
}
}
}
```

Inizia con il composable GoogleMap e l’entry point sperimentale MapEffect descritti nell’issue, quindi esamina come viene costruito KmlLayer con un GoogleMap e Context. Il lavoro è completo quando esiste un modo supportato per aggiungere KML e layer simili senza MapEffect, gestendo il caricamento lento dei layer al di fuori del thread principale.

Scritto dal modello di indicizzazione a partire dal testo della issue.