[Partition] Link label text color parameter passed is ignored

[dej611]

Describe the bug
While the textColor parameter is still accepted as valid for the linkLabel configuration object, its value is overwritten internally to the best "contrast" color against the container background color.

While the aim sounds nice, in Kibana charts embeddable cannot have a background color as Canvas requires them to be styleable somehow, so the background color is currently set to undefined/transparent. This leads to manually compute the textColor programmatically in Lens, and we land on the point above.

Expected behaviour
Respect the passed value for textColor.

Kibana Cross Issues
elastic/kibana#116777

Checklist

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[nickofthyme]

I'll verify this is fixed in #1402.