Allow filters on struct fields to be pushed down into Parquet scan (#20822)

## Which issue does this PR close?

- Related to #20603 

## Rationale for this change

This PR enables Parquet row-level filter pushdown for struct field
access expressions, which previously fell back to a full scan followed
by a separate filtering pass, a significant perf penalty for queries
filtering on struct fields in large Parquet files (like Variant types!)

Filters on struct fields like `WHERE s['foo'] > 67` were not being
pushed into the Parquet decoder. This is because `PushdownChecker` sees
the underlying `Column("s")` has a `Struct` type and unconditionally
rejects it, without considering that `get_field` resolves to a primitive
leaf. With this change, deeply nested access like `s['outer']['inner']`
will also get pushed down because the logical simplifier flattens it
before it reaches the physical plan

Note: this does not address the projection side and should not be
blocked by it. `SELECT s['foo']` still reads the entire struct rather
than just the needed leaf column. That requires separate changes to how
the opener builds its projection mask.

Author: friendlymatthew

Cargo.lock

@@ -39,6 +39,7 @@ datafusion-common-runtime = { workspace = true }
 datafusion-datasource = { workspace = true }
 datafusion-execution = { workspace = true }
 datafusion-expr = { workspace = true }
+datafusion-functions = { workspace = true }
 datafusion-functions-aggregate-common = { workspace = true }
 datafusion-physical-expr = { workspace = true }
 datafusion-physical-expr-adapter = { workspace = true }