[SPARK-20535][SPARKR] R wrappers for explode_outer and posexplode_outer

## What changes were proposed in this pull request?

Ad R wrappers for

- `o.a.s.sql.functions.explode_outer`
- `o.a.s.sql.functions.posexplode_outer`

## How was this patch tested?

Additional unit tests, manual testing.

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

Closes #17809 from zero323/SPARK-20535.