Use the hashed v2 lookup API for 3PIDs

[lampholder]

[lampholder]

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).

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Edited by @jryans to clarify that only the IS is involved.

[jryans]

Riot Web recently added a usage of v1/bulk_lookup to support many lookups in Settings, but this can be replaced by v2/lookup (defined by this hashing MSC) which supports many lookups in one request.