We'd like a mechanism by which a client can add "unsigned" data to their device
keys, and have it be accessible by other clients involved in E2EE discussions.
Most of this actually already works; the bit that doesn't is that *client-side*
`/keys/query` strips out any "unsigned" data from the `/keys/upload`
body. (Server-side `/keys/query` follows a different codepath and is fine).
This commit adds an experimental option which modifies client-side
`/keys/query` so that `unsigned` data is preserved.
37d7c506d2