Replace AuthenticationExtensionsAuthenticatorInputs with CDDL

[emlun]

This replaces the unused WebIDL type AuthenticationExtensionsAuthenticatorInputs with formally defined CDDL extensions point for authenticator extension in/outputs. It doesn't precisely encode that entry keys must be extension identifiers, but the client counterparts also don't, and the prose definition does state this requirement and illustrate it with an example.

Fixes #1435.

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[equalsJeffH]

LGTM thx @emlun, one modest suggestion...

[equalsJeffH]

on 2020-06-17 call: @selfissued will review this week.

[selfissued]

I'm OK with the change to CDDL but I think it would be more helpful to people comparing level 1 and level 2 to leave the type names the same. Therefore please change extensionInputs back to AuthenticationExtensionsAuthenticatorInputs and change extensionOutputs back to AuthenticationExtensionsClientOutputs.

After that change, I'll approve the PR.

[emlun]

Thanks @selfissued - I've renamed the extensionInputs type to AuthenticationExtensionsAuthenticatorInputs, and extensionOutputs to ...AuthenticatorOutputs rather than AuthenticationExtensionsClientOutputs since the latter still remains as a different WebIDL type. The group sockets are still named $$extensionInput and $$extensionOutput, though - would you like me to rename them to AuthenticationExtensionsAuthenticatorInput and ...Output (singular) also?

I also took the opportunity to add some more cross-references and explicitly spell out to use the extension ID as the entry key for the group plugs.

[equalsJeffH]

Looks even better to me :)

[selfissued]

Looks good now - thanks