Search - add case insensitive flag to term, prefix and wildcard queries

[markharwood]

Similar to #59441 we should add an option for a case_insensitive flag to term prefix and wildcard queries.

Relates to #61162

[elasticmachine]

Pinging @elastic/es-search (:Search/Search)

[markharwood]

For flattened object field types it's not clear if a case_insensitive term query should apply to just the value or the field name too.