Define max filesize of accepted _redirects file

[lidel]

This library should have a sane default around maximum allowed size of the _redirects file, to remove DoS vector at places where arbitrary CID root can be loaded (e.g., subdomain gateways)

There is some prior art in https://gitlab.com/gitlab-org/gitlab-pages/-/issues/472, according to their use case a very big website with ton of redirect rules should still be no more than 64 KiB.

I propose we use this as a hard limit for now and error Parse when more than 64 KiB of rules is read.
This can be a lazy-check performed as we go, this is just a precaution at the library level, in case someone prepares fake dag-pb node with spoofed size value to try to work around any size check done in Kubo.

If this sounds fine, please:

• add test that confirms library errors when file is bigger than the limit.
• add a paragraph about the maximum size to IPIP: Gateway _redirects File specs#290 so alternative implementations do the same.