Limit transaction cost

[steve-chavez]

I would like to have a plan_filter.transaction_cost_limit setting, with a behavior similar to plan_filter.statement_cost_limit but this would limit the sum of the statements costs in a transaction, this is useful as a security measure for webapps that expose endpoints that enable batch operations in a single transaction.

My idea is to limit misbehaving clients that could specify a large number of queries through the batch interface and reject their requests mapping the plan cost limit exceeded error to a HTTP 429 Too Many Requests status code; there are mechanisms at the application level to defend against this kind of misbehavior but I think it would be simpler/cleaner to enforce it at the database level.

Any thoughts about this? If it seems like a reasonable feature, I could work on a PR.