Is your feature request related to a problem? Please describe.
Allow sortIndexAccessor to define the accessor by which to sort a bucket element.

Describe the solution you'd like
Add this prop to SeriesSpec and implement sorting by the defined accessor.

Describe alternatives you've considered

Additional context
Original discussion here: #155 (review)

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