Bulk API should support documents encoded in JSON as well as encoded in SMILE

Currently the High Level Rest Client supports bulks API to send documents encoded either in JSON or SMILE as stated in the [docs](https://www.elastic.co/guide/en/elasticsearch/client/java-rest/current/java-rest-high-document-bulk.html):

```
The Bulk API supports only documents encoded in JSON or SMILE. Providing documents in any other format will result in an error.
```

I think this client should support it as well.

It is important to note that this is not only a wire concern as it has implication in how Elasticsearch will store the documents. The format the documents are sent determines how they are stored in `_source` which it has implication in index size and performance when fetching fields.