Edit.Poly middle marker custom icon

[josebui]

• [x] I'm reporting a bug, not asking for help
• [x] I've looked at the documentation to make sure the behaviour is documented and expected
• [x] I'm sure this is a Leaflet Draw code issue, not an issue with my own code nor with the framework I'm using (Cordova, Ionic, Angular, React…)
• [x] I've searched through the issues to make sure it's not yet reported

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How to reproduce

Right now there is now way to customize the icon for middle markers, just for markers in general, this is important for the users so that it can be even clearer the intent of middle markers.

Middle Marker creation: https://github.com/Leaflet/Leaflet.draw/blob/65d7866b14e926b72465d0077809683a77344019/src/edit/handler/Edit.Poly.js#L415

Marker creation: https://github.com/Leaflet/Leaflet.draw/blob/65d7866b14e926b72465d0077809683a77344019/src/edit/handler/Edit.Poly.js#L236

What behaviour I'm expecting and which behaviour I'm seeing

I would expect an option to define the opacity and icon for middle markers.