[SPARK-17629][ML] methods to return synonyms directly

## What changes were proposed in this pull request?
provide methods to return synonyms directly, without wrapping them in a dataframe

In performance sensitive applications (such as user facing apis) the roundtrip to and from dataframes is costly and unnecessary

The methods are named ``findSynonymsArray`` to make the return type clear, which also implies a local datastructure
## How was this patch tested?
updated word2vec tests

Author: Asher Krim <akrim@hubspot.com>

Closes #16811 from Krimit/w2vFindSynonymsLocal.