Use the hashed v2 lookup API for 3PIDs

[jryans]

Cloned from element-hq/element-web#10556.

This is being specced in MSC2134.

There's a GET /_matrix/identity/v2/hash_details endpoint on the IS that riot can call to see which hashing algorithms the server supports - Riot should use sha256 if the server supports it, otherwise it should use none if that's all the server supports (we should think about whether none requires any sort of special warning, though we only expect it to be used in niche cases).