New api designed for use by apps like Kibana for auto-complete use cases.
A search string is supplied which is used as prefix for matching terms found in a given field in the index.
Supported field types are keyword, constant_keyword and flattened.
A timeout can limit the amount of time spent looking for matches (default 1s) and an `index_filter` query can limit indices e.g. those in the hot or warm tier by querying the `_tier` field
Closes#59137
73e0662f09