[SPARK-19899][ML] Replace featuresCol with itemsCol in ml.fpm.FPGrowth

## What changes were proposed in this pull request?

Replaces `featuresCol` `Param` with `itemsCol`. See [SPARK-19899](https://issues.apache.org/jira/browse/SPARK-19899).

## How was this patch tested?

Manual tests. Existing unit tests.

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

Closes #17321 from zero323/SPARK-19899.