Add Iceberg samples

[ZacBlanco]

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[ZacBlanco]

From the original thread from @aaneja:

Some questions I had -
What would be the schema for the $samples table ? Would there be any extra columns besides the ones from the underlying table

The schema should exactly map to the real table. Any updates to the real table's schema would also happen on the sample table.

Would we store some metadata for the $samples table that could be used to identify -
What table sampling method was used to seed the sample
When was the sample last updated w.r.t to the changelog

This was not mentioned in the RFC, but our prototype implementation included this info. We stored it in the iceberg table properties. I will add this to the RFC