Update icon generation command to use source assets

[joshblack]

We currently use our optimized assets when generating icons in Sketch and this is causing several rendering issues, such as #4053 and #5568. This also causes some issues on website where people are downloading icons and putting them into Sketch.

As a result, we'd like to move towards using our source assets to generate Sketch symbols. This work will include:

• Authoring an extension to our metadata package to support adding asset source information to metadata
• Including this extension in generated assets for the plugin
• Update the plugin command to use the source asset instead of taking the build info and generating the source from that descriptor

This work will also include:

• Coordinating with website around the addition of this extension and working with them to see how we can expose this in the download as opposed to the current solution