[SPARK-18702][SQL] input_file_block_start and input_file_block_length

## What changes were proposed in this pull request?
We currently have function input_file_name to get the path of the input file, but don't have functions to get the block start offset and length. This patch introduces two functions:

1. input_file_block_start: returns the file block start offset, or -1 if not available.

2. input_file_block_length: returns the file block length, or -1 if not available.

## How was this patch tested?
Updated existing test cases in ColumnExpressionSuite that covered input_file_name to also cover the two new functions.

Author: Reynold Xin <rxin@databricks.com>

Closes #16133 from rxin/SPARK-18702.