[Feature] enable `boundaryGap` for category axis

[xdbobname]

What problem does this feature solve?

I'd like to align ending axis labels with the boundary, but the boundaryGap not suitable for category axis chart. I suggest extend boundaryGap api，passed a fixed value to describe onBand margin. Such as [10,20] means, 10px for left margin, 20px for right margin.

What does the proposed API look like?

I suggest extend boundaryGap api，and pass absolute value to describe onBand margin. Such as [10,20] means, 10px for left margin, 20px for right margin