Add Iceberg concurrent-writers unit and IT tests

[ashvina]

At this point, I do not know of any scenario which may require "in-flight" checks for Iceberg. The commit time (end of transaction) seems to be creating write serializability. A failed writer, in case of version conflict with concurrent writers, retries by "rebasing" on the successful writer's commit. I can add a few concurrency tests to validate this and document the concurrency scenarios which should work with OneTable.

Originally posted by @ashvina in https://github.com/onetable-io/onetable/pull/129#discussion_r1374981369