Allow showlegend for 3d Surface plots

For Surface plots it is currently not possible to show the legend.
If layout.itemclick and layout.doubleclick would default to a radio-button-like behavior (see #6617 ) for Surface plots, the legend would be very useful.

For use-case see my antenna-plot at
https://blog.runtux.com/content/2022/12-el-1deg-plot3d.html
source at https://github.com/schlatterbeck/plot-antenna

This currently uses a workaround: It adds another Scatter plot and ties the legend items of the Surface plot to the legend items of the Scatter plot using legendgroup. Then we can make the legend visible.

Not allowing showlegend with Surface plots seems an arbitrary restriction.

Inizia tracciando come i grafici Surface gestiscono showlegend e come sono definiti layout.itemclick e layout.doubleclick, usando issue #6617 per il comportamento proposto simile ai pulsanti radio. Confronta il comportamento attuale di Surface con il workaround di Scatter che usa legendgroup. Il lavoro è completato quando gli elementi della legenda di Surface possono essere mostrati e rispondono in modo coerente al comportamento di clic previsto.