Integrate nested schema support into DefaultSchemaAdapter

extend DefaultSchemaAdapter to handle nested struct fields
remove NestedStructSchemaAdapter module
update re-exports and tests
adjust nested stats mapping test

Author: kosiew

datafusion/core/src/datasource/mod.rs

@@ -42,7 +42,6 @@ pub use datafusion_catalog::default_table_source;
 pub use datafusion_catalog::memory;
 pub use datafusion_catalog::stream;
 pub use datafusion_catalog::view;
-pub use datafusion_datasource::nested_schema_adapter;
 pub use datafusion_datasource::schema_adapter;
 pub use datafusion_datasource::sink;
 pub use datafusion_datasource::source;

datafusion/datasource/src/mod.rs

@@ -38,7 +38,6 @@ pub mod file_scan_config;
 pub mod file_sink_config;
 pub mod file_stream;
 pub mod memory;
-pub mod nested_schema_adapter;
 pub mod schema_adapter;
 pub mod sink;
 pub mod source;