Strongly typing client extension inputs and outputs

I've created the PR to strongly type client extension inputs and outputs. Please have a look at https://github.com/w3c/webauthn/pull/765.

Among other things, this will enable straightforward support of the AppID extension.  For instance, the AppID definition will include:
Client extension input

A single USVString specifying a FIDO AppID.

partial dictionary AuthenticationExtensionsClientInputs<https://api.csswg.org/bikeshed/#dictdef-authenticationextensionsclientinputs> {

  USVString<https://heycam.github.io/webidl/#idl-USVString> appid;

};

This is derived from this base class:
dictionary AuthenticationExtensionsClientInputs {
};

Other changes are analogous.  Please review and let's get this checked in so extensions can be easily supported in browsers.

                                                       Thanks all,
                                                       -- Mike