`geom_spoke`: add a parameter that controls the unit for the `radius`

The radius of the spokes can be interpreted differently in different plots:

1. The radius units are bound to the axis units.
Example:
```python
wind_data = {
'x': [0, 0, 0, 1, 1, 1, 2, 2, 2],
'y': [0, 1, 2, 0, 1, 2, 0, 1, 2],
'speed': [0.84, 0.89, 0.92, 0.89, 0.94, 0.97, 0.92, 0.97, 0.99],
'direction': [0.78, 0.72, 0.69, 0.84, 0.78, 0.75, 0.87, 0.81, 0.78],
}

ggplot(wind_data) + \
geom_spoke(aes('x', 'y', radius='speed', angle='direction'),
arrow=arrow(type='closed', angle=15))
```

This is exactly how Lets-Plot works now.

2. The radius has its own units.
Example:
```python
volcano_data = {
'lat': [13.60, 14.88],
'lon': [41.80, 39.92],
'eruptions_count': [3, 2],
}

ggplot(volcano_data, aes('lon', 'lat')) + \
geom_livemap(zoom=8, location=[40.86, 14.24]) + \
geom_point(size=5, color='red') + \
geom_spoke(aes(radius='eruptions_count'), angle=0.79, color='red')
```

Here we want to be able to change the length of the spokes arbitrarily using a scale (see also [issue #1070](https://github.com/JetBrains/lets-plot/issues/1070)).
To do this, we need to be able to change the units of the radius.