BATCH-2041: Add support for reading excel files

This commit adds support for reading Excel files. Both JExcel and Apache POI are supported. Support for reading Excel files has been designed similar to the support for reading flat files.

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Looking for the best solution here. Wondering if creating a class like `SheetInfo` or `SheetMetaData` and pass that instead would be better. Or maybe wrap the information on the sheet together with the `String[]` in a wrapper `RowInfo` or `RowData` or something like that.