Feature Req: create reverse lookup for identity links

[wslyvh]

Description

I'd like to do a reverse lookup from alsoKnownAs, so it would be possible to find an account/DID based on someone's Github or Twitter username.

Technical Information

I initially asked this on Ceramic Discord, where Spencer suggested the following (and to create a feature request here);

"if we changed the way identity-link services worked, we could add a deterministic tile document as part of creating the link that would provide the reverse-link, similar to what caip10link is to the cryptoAccounts definition. This would be a good feature request for identity-link services"