Updates the metadata types to use the latest version of the specs,
getting rid of some confusion around some previous non-conformance
Introduces modules for WebIDL and UAF standards, whose type
definitions can be used by both the webauthn and metadata spec types
Working towards the same approach for metadata decoding/encoding as
for webauthn by having a module that just contains the direct
representations of the spec types and deriving automatic To/FromJSON
instances for those via deriving-aeson.