Add legend to PNG snapshot

[rshen91]

Is your feature request related to a problem? Please describe.
Currently, the PR #451 takes a PNG or msBlob of the canvas but this does not yet include the legend if it is present. This issue is opened to track adding the legend to the PNG.

Additional context
PR #451 was implemented to get the PNG functionality but including the legend is a good section iteration improvement.

Kibana Cross Issues

Checklist

• this request is checked against already exist requests
• every related Kibana issue is listed under Kibana Cross Issues list
• kibana cross issue tag is associated to the issue if any kibana cross issue is present

[streamich]

Some ideas for extending the current snapshot PNG generation:

• It could always generate Blob (instead of now in IE 11 it generates Blob but in other browsers a Data URL).
• Optionally, a legend could be rendered under the chart, this way there could be as many items in the legend as needed.
• Optionally, a title could be rendered at the top.
• Optionally, extra whitespace padding could be added around the chart and legend to look a bit nicer when the PNG is shared somewhere.