[structurizr-export] Support local icon files and embeded data images

[mwz]

Description

Currently the AbstractPlantUMLExporter only allows remote images to be included in the exported diagrams; see:

java/structurizr-export/src/main/java/com/structurizr/export/plantuml/AbstractPlantUMLExporter.java

Lines 226 to 228 in e328063

	protected boolean elementStyleHasSupportedIcon(ElementStyle elementStyle) {
	return !StringUtils.isNullOrEmpty(elementStyle.getIcon()) && elementStyle.getIcon().startsWith("http");
	}

I'd like to propose to support local images as well as data URIs to bring the functionality in line with the image support in structurizr.

Structurizr already supports local pngs, jpgs and svgs as well as image data URIs, so this change would make the export functionality consistent with the rest of the project.

Priority

Low (I'm willing to make a pull request - please confirm approach first)

More information

As far as I know C4 Plantuml supports local files and data URIs as image sprites, so this should work, but I've only tested it with svgs.