Expose MiniatureShapeSize on ISeries interface

[MithrilMan]

Exposing MiniatureShapeSize on ISeries interface (that's already implemented in the Series class) allows to customize the theme using HasRuleForAnySeries or specialized extensions like HasRuleForLineSeries,

[MithrilMan]

On my local solution I disabled most of the project because I'm interested in Blazor (and Winform to test it fast)
Using this in Program.cs file (WinFormsSample.Program)

LiveChartsCore.LiveCharts.Configure(config => config
            .AddSkiaSharp()
            .AddDefaultMappers()
            .AddLightTheme(theme => theme
                .HasRuleForAnySeries(series =>
                {
                    series.MiniatureShapeSize = 5;
                })
                )
            );

gave this result on the General/Legend sample (and every other sample using Legends)

as opposed to the original one

[beto-rodriguez]

Thanks for the PR!