Support for multiple statements in a single query

[mildbyte]

Several examples in the docs (e.g. https://www.splitgraph.com/docs/seafowl/guides/baking-dataset-docker-image#building-a-docker-image that uses #39) require being able to pass multiple queries in a single command. We currently don't allow that (copied from DF): https://github.com/splitgraph/seafowl/blob/main/src/context.rs#L551-L558

The user could always send their queries one-by-one (which is why this is a P2, if we don't have time for it, we can change the docs to mention that), but we can use this as a basis to create implicit mini-transactions, i.e.:

CREATE TABLE staging AS (...);
DROP TABLE production;
ALTER TABLE staging RENAME TO production

which would get executed by Seafowl inside of a transaction on the catalog. The effect would be that other queries will always see the production table, even while it's being recreated.

The semantics would be:

• return the output of the last query
• for queries that aren't the last one, only execute writing queries (no point in executing RO queries if we won't see their results)