[Iceberg] Populate planning statistics from table metadata

[gabotechs]

Part of #599.

Populate planning-time statistics for IcebergDataSource from the Iceberg table and selected snapshot.

Essentially, implementing this TODO comment in the code:

datafusion-distributed/iceberg/src/data_source.rs

Lines 252 to 255 in 72203a0

	// TODO: Implement planning time statistics for this DataSource.
	// At this point, we have information about the iceberg::table::Table which we are about
	// to read, so maybe there's something we can get from there.
	Ok(Arc::new(Statistics::new_unknown(&self.schema)))

The snapshot summary and file metadata may provide row-count and byte-size estimates without planning all scan tasks.

[gabotechs]

Addressed in #676.

Opened this as a follow up:

• [Iceberg] Add per-column statistics to the IcebergDataSource #681

[gabotechs]

Thanks for addressing this @sandugood!