#121261, #122607) (#122805)

* ESQL: Support for _index metadata field in CsvTests (#121261)

* ESQL: Support for _index metadata field in CsvTests

* Extract INDEX constant to MetadataAttribute

* Add comment on capability

* ESQL: Initial support for unmapped fields (#119886)

This PR adds initial support for unmapped fields, using the INSIST clause. For starters, this unmapped fields without a cast.
Note that the INSIST keyword is potentially a placeholder, as the method of defining an unmapped field might change in the future, e.g., use a special magic function. As this is currently under development, the actual syntax is INSIST_🐔.

First stage of #120072.

Specifically, the following features are implemented in this PR:

* Support for INSIST keyword without a cast. In particular, if the type being INSISTed upon is mapped to anything other than KEYWORD, it will result in an InvalidMappedField. There is no support for union type resolution on top of INSIST. Future PRs will handle these conflicts. There is support for multiple parameters, or INSIST on top of INSIST which is equivalent.
* Enforcing that INSIST must always be on top of a FROM or another INSIST. While this may change in the future, e.g., handling cases like `FROM foo | EVAL x = 1 | INSIST bar` will not be done in this PR, as it makes handling INSIST too complicated.