[Feature request] Set font size for legendgroup titles in layout

[stephprobst]

It would be great, if we could set the font size for all legendgroup titles in the layout, similar to the font size for the overall legend title. Currently, the font size of the legendgroup title has to be set for each trace individually.

[archmoj]

How about legendgrouptitlefont and hovergrouptitlefont (for the case of unified hover) as the attribute names?

[stephprobst]

Both sound good to me. Thanks!