[SPARK-12398] Smart truncation of DataFrame / Dataset toString

When a DataFrame or Dataset has a long schema, we should intelligently truncate to avoid flooding the screen with unreadable information.
// Standard output
[a: int, b: int]

// Truncate many top level fields
[a: int, b, string ... 10 more fields]

// Truncate long inner structs
[a: struct<a: Int ... 10 more fields>]

Author: Dilip Biswal <dbiswal@us.ibm.com>

Closes #10373 from dilipbiswal/spark-12398.