Allow users to specify index of the header row in sheet config

[bastienboutonnet]

As seen in #292 and #293 sheets might not have the first row as their header. In that case sheetwork currently fails or when #293 will be implemented it will throw and error that the first row is empty.

In some cases users who are responsible for the ingest may not have rights to edit the sheet or it might not even be handy to do so. In that case we should allow the user to pass a setting in the sheet.yml config that looks something like this:

  - sheet_name: sheet_with_wonky_header
    sheet_key: xxxxxxxxxxxx
    worksheet: sheet1
    header_index: 2 # where the integer corresponds to the row as seen on the GoogleSheet
    target_schema: sand
    target_table: bb_test_sheetwork

NOTE: GoogleSheet indeces do not start from 0 so we need to make sure we handle the conversion under the hood.