Ability to pull Column Keys from a different row (second/third row, etc)

Problem/Motivation

Sometimes when structuring our export spreadsheets, we include groupings in the first row and the second row contains the actual column headings. See the screenshot below:

It would be great if we could pull the headings from the second row and skip the first one entirely.

Proposed resolution

The Migrate Spreadsheet module provides the ability to specify the header row as a number, i.e. "2" would skip the first row. This comes from the migration YAML file (see code):

  # The row where the header is placed, if any. If this configuration is missed,
  # there's no table header and the spreadsheet columns (A, B, C, ...) will be
  # automatically used as table header. If the table header is on the first row,
  # this configuration should be 1. The header cell values will act as column
  # names. The value of 2 means that the table header is on the second row.
  header_row: 2

This value then gets pulled out in the Migrate source plugin.

User interface changes

None

API changes

No changes, just additions.