Partition: add border color configurability

[monfera]

Describe the solution you'd like
Request from @cchaos: be able to set the color of pie/sunburst sector borders, which are currently always white:

elastic-charts/src/chart_types/partition_chart/renderer/canvas/canvas_renderers.ts

Line 222 in 544b7cc

ctx.strokeStyle = 'white'; // todo make it configurable just like sectorLineWidth

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

[markov00]

🎉 This issue has been resolved in version 18.2.0 🎉

The release is available on:

• GitHub release
• npm package (@latest dist-tag)

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