Logos and icons always should be in vector format so you are able to scale them without any loss in quality. If you don’t have a vector format like svg you could just use the png-image without the advantages of svgs or you need to recreate that logo with figmas pen tool and export as svg. I would always prefer a vector.

How would you import an icon in .svg format?

Just by drag ‘n’ drop it onto the canvas or by File → Place image.

How to add it to your plugin I don’t know.