Add support for reading Theta sketch Puffin file

[ebyhr]

Rationale for this change

apache-datasketches-theta-v1 was already listed as a valid blob type in PuffinBlobMetadata and BlobMetadata, but pyiceberg had no way to read the actual sketch data. This implements deserialization so callers can access NDV estimates stored in statistics Puffin files written by engines like Spark.

• https://iceberg.apache.org/puffin-spec/#apache-datasketches-theta-v1-blob-type

Are these changes tested?

Yes.

Are there any user-facing changes?

Yes. New pyiceberg/table/theta_sketch.py with ThetaSketch class and theta_sketches_from_puffin_file function. New optional dependency datasketches under the datasketches extra.

[github-actions]

This pull request has been marked as stale due to 30 days of inactivity. It will be closed in 1 week if no further activity occurs. If you think that's incorrect or this pull request requires a review, please simply write any comment. If closed, you can revive the PR at any time and @mention a reviewer or discuss it on the dev@iceberg.apache.org list. Thank you for your contributions.

[github-actions]

This pull request has been marked as stale due to 30 days of inactivity. It will be closed in 1 week if no further activity occurs. If you think that's incorrect or this pull request requires a review, please simply write any comment. If closed, you can revive the PR at any time and @mention a reviewer or discuss it on the dev@iceberg.apache.org list. Thank you for your contributions.