Align registries draft and WebAuthn draft and address extension issues

At the F2F, I took an action item to review the extensions mechanism to make sure that was well-defined.  I reviewed all the general-purpose extensions language, had a conversation with Jeff Hodges about it, and have now created a pull request that make it better defined.  Along the way, I also brought the IANA Registries draft into sync with the WebAuthn draft, in preparation for submitting it to the IETF on Monday (at the start of IETF 98 in Chicago).

Please see https://github.com/w3c/webauthn/pull/386 for these changes.  I reviewed the thrust of the changes with Jeff verbally but he has not yet reviewed the actual text.

                                                                -- Mike