Implement the functionality to translate the
`field IN (value1, value2,...)` expressions to proper Lucene queries
or painless script or local processors depending on the use case.
The `IN` expression can be used in SELECT, WHERE and HAVING clauses.
Closes: #32955
Split function section into multiple chapters
Add String functions
Add (small) section on Conversion/Cast functions
Add missing aggregation functions
Enable documentation testing (was disabled by accident). While at it,
fix failing tests
Improve spec tests to allow multi-line queries (useful for docs)
Add ability to ignore a spec test (name should end with -Ignore)