Intraday heatmap should support automatically adding sunrise/sunset lines

Currently, the sunrise/sunset lines need to be added separately which is a hassle.

I am imagining that two additional parameters can be added (``latitude``/``longitude``), which defaults to None (optional) and when they are both specified, then sunset/sunrise lines are added.

```python
fig, ax = plt.subplots(figsize=(6, 3))

_ = solarpy.plotting.plot_intraday_heatmap(
time=data.index,
values=data["ghi"],
cmap=cmap,
norm=norm,
colorbar_label="GHI [W/m²]",
ax=ax, # pass in existing axes
)

# Overlay sunrise and sunset times
sun_rise_set = pvlib.solarposition.sun_rise_set_transit_spa(
pd.date_range(data.index.min(), data.index.max(), freq="1d"),
meta["latitude"],
meta["longitude"],
)

sunrise = (
sun_rise_set["sunrise"] - sun_rise_set["sunrise"].index.normalize()
).dt.total_seconds() / 3600

sunset = (
sun_rise_set["sunset"] - sun_rise_set["sunset"].index.normalize()
).dt.total_seconds() / 3600

ax.plot(sunrise, c="r", linestyle="dashed", lw=1.5, alpha=0.7)
ax.plot(sunset, c="r", linestyle="dashed", lw=1.5, alpha=0.7)
```

Start at the plot_intraday_heatmap entry point and inspect how its parameters and axes are handled. Add optional latitude and longitude inputs, then verify that supplying both produces sunrise and sunset lines while omitting either preserves the current plot behavior. Check the existing plotting tests or test suite to confirm the expected figure output.

Written by the indexing model from the issue text.