feat: ⚡️ add 'raw_discourse_categories' asset

[DistributedDoge]

Creates a single asset that builds a table, where each row is a Discourse category like say "Proposals" or "Grants" on some Discourse instance. Not all Discourse instances give us listings of topics, but all of them yield stats (e.g. number of posts in category)

• Currently scans /categories.json endpoint for Giveth, Gitcoin, Arbitrum, Optimism , more forums can be added
• Use source column to identify Discourse from which specific category was taken.

Further work will be:

• add dbt model
• build downstream model that fetches topic bodies for interesting categories for each forum (e.g. fetch PROPOSALS/GRANTS from both Gitcoin and Giveth)

I think data is interesting as-is and model is stable so it should be ready to merge!