[Feature] Configurable priority level for displaying X axis labels

### What problem does this feature solve?

In some charts, certain labels, e.g the start of the year or month should be prioritized to show over others.

### What does the proposed API look like?

`xAxis.axisLabel.interval` : `(index:number, value: string) => number`

The higher the number, the higher the priority to show over lower priority ones when `hideOverlap: true`.
e.g.

```ts
const options = {
xAxis: {
type: "category",
interval: (index, value) => {
if (firstDayOfYear(value)) {
return 2
}
if (firstDayOfMonth(value)) {
return 1
}
return 0
},
}
...
}
```