plot_sky scatter labels for single target in time

When doing something like

```python
from astroplan.plots import plot_sky
from astroplan import time_grid_from_range
import astropy.units as u

start_time = Time("2024-01-01 00:00:00")
time_grid = time_grid_from_range([start_time, start_time+30*u.day*12], time_resolution=30*u.day)

plot_sky(target_source, observer, time_grid, ax=None, style_kwargs=None, north_to_east_ccw=False, grid=True, az_label_offset=0*u.deg, warn_below_horizon=False, style_sheet=None)
```

the timing information is missing from the plot (in this case it would be a small number from 1 to 12 months, but in the case of an arbitrary amount of time it would be a finer array)

I did not find a simple way from astroplan API to do it, so I would propose to do it in the form of a (optional) colorbar which with a time scalar mappable maps time to color.

Start with the astroplan.plots.plot_sky entry point and reproduce the single-target example using its time grid. Check how the current scatter labels are produced and confirm that timing is absent; done means an optional colorbar maps the supplied times to colors and conveys the timing information for arbitrary intervals.

Written by the indexing model from the issue text.