Replace offsetX and offsetY with padding in ValueLabels

[markov00]

Is your feature request related to a problem? Please describe.
The current offsetX and offsetY property used in the bar value labels, is used to push the label far from the geometry edges. It basically represents the same behavior of a padding but without the hassle of configuring the offset based on the rotation.

Describe the solution you'd like
We can replace the offsets with a single padding value, this should cover the general cases and it can also improve situations like the following where the label is centered by has an offset of 2px. The label on the blue bar should be anyway centered, the padding can be ignored to correctly fill all the text in the center, or completely remove the value if it doesn't fit in width together with his padding.

Additional context
see elastic/eui#4845