[SPARK-20830][PYSPARK][SQL] Add posexplode and posexplode_outer

## What changes were proposed in this pull request?

Add Python wrappers for `o.a.s.sql.functions.explode_outer` and `o.a.s.sql.functions.posexplode_outer`.

## How was this patch tested?

Unit tests, doctests.

Author: zero323 <zero323@users.noreply.github.com>

Closes #18049 from zero323/SPARK-20830.