Add index_prefixes to Text property mapping

[russcam]

Relates: elastic/elasticsearch#29067 and elastic/elasticsearch#28994
Documentation: https://www.elastic.co/guide/en/elasticsearch/reference/6.3/text.html#index-prefix-config

Add support for index_prefixes configuration on text mapping, to speed up prefix searches

[russcam]

Closing this as #3362 has been merged into 6.3