This implements support for MSC2732[1], fallback keys. Only support to
upload and get notifications about fallback keys via `/sync` is
implemented.
[1]: https://github.com/matrix-org/matrix-doc/pull/2732
Uploaded objects holding public keys tend to have the object signed by
the very same key that is part of the object. Users that download such
public keys are expected to verify the signature to confirm that the
public key and its surrounding fields in the object have not been
manipulated with.
The SignedKey, DeviceKeys, and CrossSigningKey structs
perform a lossy variant of deserialization, which removes the ability to
verify signatures of the aforementioned objects.
This patch changes the response types to a Raw variant, which let's the
user decide how to deserialize.
