feat: hivemind ETL discourse summary retriever

We need to define the discourse platform summary retriever based on the definitions in the card
https://www.notion.so/rndadocs/Hivemind-Forum-platform-retriever-Milestone-3b-2013c8a8f1e243bc8138e523f49656b9

The output would be the summary nodes' metadata to do a metadata filtering on raw data.