add a cost_threshold global configuration option

[ethanpailes]

SQL has an EXPLAIN statement. Let's use it. We can allow users to configure a max allowable cost for a query. Any query with a higher cost will need to have the cost_threshold = <higher value> option set for it or we generate a hard error.

One potentially tricky part of this is that we typically use an empty database for codegen, and the costs will differ based on the actual contents of the database. For this reason we might want to add a "check mode" so that people can avoid running these checks against their prod db in their main build pipelines.