* Enhances exists queries to reduce need for `_field_names`
Before this change we wrote the name all the fields in a document to a `_field_names` field and then implemented exists queries as a term query on this field. The problem with this approach is that it bloats the index and also affects indexing performance.
This change adds a new method `existsQuery()` to `MappedFieldType` which is implemented by each sub-class. For most field types if doc values are available a `DocValuesFieldExistsQuery` is used, falling back to using `_field_names` if doc values are disabled. Note that only fields where no doc values are available are written to `_field_names`.
Closes#26770
* Addresses review comments
* Addresses more review comments
* implements existsQuery explicitly on every mapper
* Reinstates ability to perform term query on `_field_names`
* Added bwc depending on index created version
* Review Comments
* Skips tests that are not supported in 6.1.0
These values will need to be changed after backporting this PR to 6.x
99aca9cdfc