Proper middleware support

FYI the convex-helpers way of doing this currently is to use onSuccess like so:

const actionWithFlush = customAction(internalAction, {
  args: {},
  input: async () => {
    const logger = createLogger();
    return {
      ctx: { logger },
      args: {},
      onSuccess: ({ ctx, args, result }) => {
        logger.flush(result);
      },
    };
  },
});

const myAction = actionWithFlush({
  args: {},
  handler: async (ctx) => {
    ctx.logger.log("foo");
  },
});

Note: this won't fire if your action times out or otherwise exits un-cleanly, so putting a try/catch around it and having a Promise.race with a 9min timer and flushing your errors before re-throwing would be my recommendation. Acknowledged that (especially for actions) having a "finally" clause would be more ergonomic here.