[webauthn] Extensions need to define how their parameters convert to/from CBOR

jcjones has just created a new issue for https://github.com/w3c/webauthn:

== Extensions need to define how their parameters convert to/from CBOR ==
Extension definitions need to be more thorough, as @bzbarsky [points out in the discussion for issue #624](https://github.com/w3c/webauthn/issues/624#issuecomment-335853416). In particular, for each extension -- including the example -- we need two sections:

1. How to encode the `AuthenticationExtensions` data structure  into CBOR for the wire protocol
2. How to decode the resulting extension CBOR into the `AuthenticationExtensions` data structure

Without these sections, it's under-specified how to actually implement these extensions.

Please view or discuss this issue at https://github.com/w3c/webauthn/issues/626 using your GitHub account