Support for search-as-you-type datatype

[rickardp]

NEST/Elasticsearch.Net version:
7.2.0, master
Elasticsearch version:
7.2.0
Description of the problem including expected versus actual behavior:
The search_as_you_type datatype is not available in NEST.

Steps to reproduce:

1. Try to use search_as_you_type data type or bool_prefix multi match type

The examples in https://www.elastic.co/guide/en/elasticsearch/reference/7.x/search-as-you-type.html should work with NEST.