Adds CloudBucketMount example mounting S3 bucket

[luiscape]

Adds example that shows how to use a CloudBucketMount in Modal. This example both writes and reads from an S3 mount. Both writes and reads are done in parallel using both map and starmap.

The example is similar to: https://modal.com/docs/examples/duckdb_nyc_taxi

[charlesfrye]

Made some changes, as discussed offline.

This is ready to merge once it passes in synmon -- aka once are all workers ready to run it.