This commit introduces a new execution mode for the query_string query, which
is intended down the road to be a replacement for the current _all field.
It now does auto-field-expansion and auto-leniency when the following criteria
are ALL met:
The _all field is disabled
No default_field has been set in the index settings
No default_field has been set in the request
No fields are specified in the request
Additionally, a user can force the "all-like" execution by setting the
all_fields parameter to true.
When executing in all field mode, the query_string query will look at all the
fields in the mapping that are not metafields and can be searched, and
automatically expand the list of fields that are going to be queried.
Relates to #19784
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