## What changes were proposed in this pull request?

- Adds R wrapper for `o.a.s.sql.functions.broadcast`.
- Renames `broadcast` to `broadcast_`.

## How was this patch tested?

Unit tests, check `check-cran.sh`.

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

Closes #17965 from zero323/SPARK-20726.