Add more Vega examples to the editor

[timroes]

We currently have only one example of a Vega spec loaded by default when creating a new Vega visualization. Instead we want to add a couple of more examples covering common use-cases and allow the user to load in the editor one of those examples if they want to. Design wise we currently think of having another icon beside the "settings" icon, that shows a popup allowing to chose one of those examples.

The following examples would be a good starting point:

• Simple XY-Axis chart (what we currently have)
• XY-Axis chart with split series (something that needs the data to be flatten)
• Pie Chart

[elasticmachine]

Pinging @elastic/kibana-app

[flash1293]

I think a day over day metric with arrow up/down would also be nice because it's a common use case.
Reference: https://nyurik.github.io/Vega-Trends-Graph-for-Kibana/

[stratoula]

I think it is related to this #17054