feat: Enable setting index and error documents.

[octalmage]

I'm working on static site hosting using Swarm and I noticed not having these headers listed as allowed is causing CORS errors when using bee.js:

Access to XMLHttpRequest at 'https://swarm.o8.is/bzz' from origin 'http://localhost:3000' has been blocked by CORS policy: Request header field swarm-error-document is not allowed by Access-Control-Allow-Headers in preflight response.

I understand why some of the other headers might not be allowed, but I think these would make a lot of sense and would enable some cool workflows.

Thanks for your time!

[octalmage]

So I ended up adding two additional headers. I think we should encourage users to use their own postage but to do that gateways need to allow the swarm-postage-stamp (for uploading signed chunks).