Term Vectors: support for distributed frequencies

Adds distributed frequencies support for the Term Vectors API. A new parameter
called `dfs` is introduced which defaults to `false`.
Maybe just index more docs to make it more likely that most of the times multiple shards are involved.

Something like this:
```java
int numDocs = scaledRandomIntBetween(25, 100);
logger.info("Indexing {} documents...", numDocs);
List<IndexRequestBuilder> builders = new ArrayList<>();
for (int i = 0; i < numDocs; i++) {
```