We're working on something called Content Aggregator, which is a generic tool for fetching data from external data sources, converting data into suitable format using Node.js modules, and then storing the data into some kind of data storage, such as Elasticsearch or in this case, file system. Much like a combination of Logstash and AWS Lambda.
As a proof of concept we've written two examples, one for fetching content and images from Contentful, and one for creating markdown for Hugo. If you combine those two examples maybe it will fit your needs?
You can find the two examples here: https://bitbucket.org/aretera/contentaggregator.samples
Or read more about Content Aggregator here: https://bitbucket.org/aretera/contentaggregator/wiki/Home
It is still work in progress. Please let me know if you have any questions!