webauthn: implement proper support for extensions + validation

[pmlopes]

WebAuthn Level 2, defines a set of core extensions. While the current API does allow specifying extensions, there is no way to tell if the extension is for registration and/or authentication. It is also not possible to process and validate any extension response.

See: https://www.w3.org/TR/webauthn-2/#sctn-defined-extensions

[pmlopes]

Currently the extensions are just a map on the options object. This should be deprecated (as it's currently a NOOP feature) and we should define a codegen interface/data object for extensions:

The shape of this object should include:

• a name
• a use (registration, authentication, both)
• a value (this could be anything: boolean, String, JSONObject, ...)
• a validator function

The JavaScript companion library should also extract the extension results and post them back to the library so it can validate the responses.