Manifest files for unixfs loaded via HTTP Gateway

This is a placeholder issue for creating a spec for "content root's manifest files" which would be part of the data published on IPFS (eg. `/ipfs/CID/_something` or `/ipfs/CID/.well-known/ipfs/foo`) that provides hints to HTTP Gateway around things like redirects, content types and custom HTTP headers.

Related reading:
- [Proposal: manifest file for the IPFS gateway](https://github.com/ipfs/go-ipfs/issues/6214)
- [Proposal: native redirects config](https://github.com/ipfs/go-ipfs/issues/7392)
- [Content Type set by HTTP Gateway](https://github.com/ipfs/in-web-browsers/issues/152)

Possible reuse of:
- [x] `_redirects` (prior art in [netlify docs](https://docs.netlify.com/routing/redirects/)
- **DONE**
- IPIP: https://specs.ipfs.tech/ipips/ipip-0002/
- Specification: https://specs.ipfs.tech/http-gateways/web-redirects-file/
- Docs: https://docs.ipfs.tech/how-to/websites-on-ipfs/redirects-and-custom-404s/
- [ ] `_headers` file (prior art in [netlify docs](https://docs.netlify.com/routing/headers/#syntax-for-the-headers-file) and [cloudflare docs](https://developers.cloudflare.com/pages/configuration/headers/))
- **TBD**

Feel free to discuss/post ideas below, or open PRs with IPIP drafts that could be reviewed independently.