[SPARK-3048][MLLIB] add LabeledPoint.parse and remove loadStreamingLabeledPoints

Move `parse()` from `LabeledPointParser` to `LabeledPoint` and make it public. This breaks binary compatibility only when a user uses synthesized methods like `tupled` and `curried`, which is rare.

`LabeledPoint.parse` is more consistent with `Vectors.parse`, which is why `LabeledPointParser` is not preferred.

@freeman-lab @tdas