[I] Support inline and stack generators in GenerateExec [datafusion-comet]

["andygrove (via GitHub)" <gi...@apache.org>]

andygrove opened a new issue, #5125:
URL: https://github.com/apache/datafusion-comet/issues/5125

   ### What is the problem the feature request solves?
   
   `GenerateExec` runs natively for `explode` and `posexplode` over arrays, but the `inline` and `stack` generators fall back to Spark, taking the surrounding stage with them.
   
   Flattening nested data is a staple of ETL over semi-structured sources: `inline` is the idiomatic way to expand an array of structs into columns, and `stack` is the standard unpivot. Both commonly appear directly above a Parquet scan, so the fallback lands on exactly the stages Comet otherwise accelerates well.
   
   ### Describe the potential solution
   
   - `inline(array<struct>)` is `explode` followed by flattening the struct fields into top-level columns, so it should be able to reuse the existing native explode machinery plus a projection.
   - `stack(n, expr1, ..., exprk)` is a constant-shape row expansion and does not depend on array input at all.
   
   `explode_outer` support is tracked separately in #2838; the `_outer` variants of `inline` would follow the same null-row semantics.
   
   ### Additional context
   
   Related: #2838 (`explode_outer`), #4269 (added `posexplode` / `posexplode_outer`).
   


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