Improve bar value styles on vis_type_xy charts

[nickofthyme]

Describe the feature:
Improve the color, alignment and sizing of bar chart values in xy charts to use same defaults as Lens.

Should set these defaults on eui charts theme

Waiting on:

• Font size relative to bar size Value labels could have a font size as a ratio of the bar width elastic-charts#788
• Font color relative to bar color Change value labels colors depending on background elastic-charts#238
• Label offset defined relative to geometry Have a displayValueSettings option to dynamically set the label's offset based on the label box elastic-charts#792