Add benchmarks for Parquet struct leaf-level projection pruning (#21180)

## Rationale for this change

This PR adds benchmarks that measure the perf of projecting individual
fields from struct columns in Parquet files. #20925 introduced
leaf-level projection masking so that `select s['small_int']` on a
struct with large string fields only reads the small integer leaf,
skipping the expensive string decoding entirely

3 dataset shapes are coevered, each with ~262K rows of 8kb string
payloads: a narrow struct (2 leaves), a wide struct (5 leaves), and a
nested struct. Each shape benchmarks full-struct reads against
single-field projections

Author: friendlymatthew

datafusion/core/Cargo.toml

@@ -247,6 +247,11 @@ harness = false
 name = "parquet_struct_query"
 required-features = ["parquet"]
 
+[[bench]]
+harness = false
+name = "parquet_struct_projection"
+required-features = ["parquet"]
+
 [[bench]]
 harness = false
 name = "range_and_generate_series"