Add support for svgBlip extension
We use openpyxl at our company, and it's a lovely experience. However, one feature the base package is currently lacking is support for svgBlip (part of MS-ODRAWXML extension). It's an extension for Images that allows to specify SVG source in addition to raster data. Currently, I've written a bunch of moderately horrible monkey-patching (~70 lines) to get the job done, however, I'd like to make the code upstream-ready. Sadly that cannot really be done as a separate package, since to work, it requires changes to Blip (to serialize extLst tag properly) and SpreadsheetDrawing (maybe not strictly necessary - there might be a better place to populate the extension list - see also #1839).
If there's an interest in that and willingness to do a review, I would love to contribute my code.