Add ODS (LibreOffice Calc) format support

[chitralabs]

Feature Request: ODS Format Support

Many enterprise users on Linux use LibreOffice Calc and work with .ods files.
Currently Sheetz only handles .xlsx, .xls, and .csv.

Requested API (no change to existing API):

// Should work identically to .xlsx
List<Product> products = Sheetz.read("data.ods", Product.class);
Sheetz.write(products, "output.ods");

Implementation notes:

• Apache POI already has ODS support via org.apache.poi:poi-ooxml
• Follow the pattern of XlsxReader.java / XlsxWriter.java
• Add OdsReader.java and OdsWriter.java implementing the same interfaces
• Register the .ods extension in FormatDetector.java
• Add test resources: a sample .ods file

Difficulty: Medium (~300 lines of new code)
Great first major feature contribution!