Support for regex in completion suggesters

[selimt]

Elasticsearch completion suggesters support regex queries:

https://www.elastic.co/guide/en/elasticsearch/reference/current/search-suggesters.html#regex

Would it be possible to support that?

For now I'd have to modify the existing query dict to replace the "text" key with the "regex" key inside the suggest viewset method.

Thanks.

[selimt]

I also opened up an issue here:
elastic/elasticsearch-dsl-py#1650

Since without it this can not be easily supported.