[Feat] Add onTileLoading callback on TileLayers

Target Use Case

Atm, any sort of TileLayer is missing a callback to know when tiles starts fetching. This could help devs implement a loading indicator that could be paired with onViewportLoad / onTileLoad.

I managed to implement it like this atm

return new CustomTileLayer({
    ...
    onTilesLoading: () => setTilesLoading(mapId, layerId, true),
    onViewportLoad: () => setTilesLoading(mapId, layerId, false)
});

interface CustomTileLayerProps {
    onTilesLoading?: () => void;
}

class CustomTileLayer<DataT> extends TileLayer<DataT, CustomTileLayerProps> {
    getTileData(tile: TileLoadProps) {
        const {
            props: { onTilesLoading }
        } = this;
        onTilesLoading?.();

        return super.getTileData(tile) ;
    }
}

Proposal

new TileLayer({
    ...
    onTilesLoading: () => { /*show loading indicator here*/ },
    onViewportLoad: () => { /*hide loading indicator here*/ }
});

Start at the TileLayer getTileData entry point shown in the issue and trace how tile fetching relates to onTileLoad and onViewportLoad. Add the proposed onTilesLoading callback to TileLayer's public API, then verify that it fires when tile fetching begins and can be used with the existing viewport-load callback.