Add size option for layout.legend.itemsizing : 'constant'

[elitadanilyuk]

As of Plotly 2.32.0 and 2.33.0, the itemsizing from Parent: layout.legend does not offer an option to further customize the size of the legend symbols.

If I want my trace symbols to be smaller, the legend symbols are too small, but when I set itemsizing: "constant" the legend symbols are too large in comparison with the chart.

Example with itemsizing: "constant"

Example with itemsizing: "trace"

[archmoj]

Related #3735, #5475 and #5475 (comment).