feat: .url property on response objects

[SgtPooki]

Created based on mention from @achingbrain at #4 (comment)

After #5 we will have the .url property on responses that is the canonical URL for the requested resource.

With this PR the user can request the same resource via multiple URL formats. Should we be rewriting this URL to turn, for example, subdomain gateway requests into ipfs:// URIs?

I think we should absolutely be setting the canonical URL to the protocol we want to be natively supported in the browsers in the future. Which means, .url should always be a ipfs:// or ipns:// url.