Download Image From Native Plots

#### Is your feature request related to a problem? Please describe

Would be very useful to be able to download Wave native plots to images.

#### Describe the solution you'd like

Perhaps right clicking on the plot allows you to save it as a PNG/copy it or maybe a button below the plot download it to a file (similar to the table functionality).

#### Describe alternatives you've considered

Could maybe be done by extracting the raw HTML using a third-party library but this seems overly cumbersome.

Start by tracing how Wave native plots are rendered and compare that entry point with the existing table download functionality mentioned in the issue. Define and verify a supported way to export a native plot as an image, including the completed user interaction and output format.

Written by the indexing model from the issue text.