Add a function for Base64 encoding to the expressions

[samuel-pina]

In Vega-Lite, we don't have the image property of the image mark as we do in vega, we only have the url one. In some uses like in Deneb in Power BI, there are certain limitations for using the url property as it could be connecting with external content. If we are using base64 encoded images declared inline for the url property, we can overcome this limitation but then we cannot alter the image (for instance, a svg) dynamically as we don't have a way in vega-lite to encode them later to base64. I explain a little bit better the scenario in this post.
At the end, it is possible to change the image dynamically in vega, but it would be great if we could modify the svg in vega-lite and then encode it in base64 for using it in the url property. Would it be possible to add such function to the expression language?