HTTP Gateway Validator

> AKA checksuming payload as it arrives from untrusted gateway
> and checking if it match expected CID

### Context

See _Verifiable HTTP Gateway Responses_ at https://github.com/ipfs/in-web-browsers/issues/128

### Work

- [x] research if/how we could calculate checksum as data arrives
- Only Firefox supports [webRequest.filterResponseData()](https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/webRequest/filterResponseData) API, so it could be Firefox-only feature for now.
- [ ] research how we resolve CID of resources under a path and get insight into DAG structure
- A: additional lookup via local js-ipfs used only for that or fallback on `http://ipfs.io/api/v0/`
- B: additional HTTP headers with required metadata
- [ ] research and design UX for indicating the payload is verified
- [ ] implement streaming validator that drops connection if payload does not match expected digest

### Related Resources

- Prior art from Cloudflare: https://github.com/cloudflare/ipfs-ext/
- it also has DNSSEC validator, but that should be a separate topic
- limitation: only SHA-256 is supported in CIDs
- If browser is able to verify CID digest, it will be possible to define public, untrusted gateway as the backend without need for trusting it returns valid data
- Reproducible File Imports | https://github.com/ipfs/unixfs-v2/issues/15