WebAuthn: Add userId property to Authenticator and WebAuthnCredentials

#### Describe the feature

Depends on fixing #580 first.

#### 1. Add userId property to relevant classes

* Add `userId` property to both `Authenticator` and `WebAuthnCredentials`.
* After successful WebAuthn authentication or credential registration, the `userId` property from `Authenticator` should be included in the principal of returned `User`.

#### 2. Populating the user id

After the property is added to relevant classes, the `Authenticator` instances passed to `authenticatorFetcher` and `authenticatorUpdater` should include the user ID (once #580 is fixed) property:

* When fetching authenticators with a query:
* In `WebAuthn#createCredentialOptions()`: copy the value from the `user` object passed in as parameter.
* In `WebAuthn#authenticate()`: use the user id defined in `WebAuthnCredentials` passed in as parameter
* When inserting or updating authenticators:
* In `WebAuthn#authenticate()` for webauthn.get: use the user id defined in `WebAuthnCredentials`
* In `WebAuthn#authenticate()` for webauthn.create: use the user id from the authenticator found by credential id lookup

#### 3. Extra check in authentication

Additionally, the verification process should ensure that the `userHandle` returned in the `AuthenticationAssertionResponse` matches the `userId` property on the `Authenticator` found with credential id lookup.

The spec:

> If the user was not identified before the [authentication ceremony](https://www.w3.org/TR/webauthn/#authentication-ceremony) was initiated,
> verify that response.[userHandle](https://www.w3.org/TR/webauthn/#dom-authenticatorassertionresponse-userhandle) is present, and that the user identified by this value is the owner of credentialSource.

#### Use cases

This would allow Relying Party to index authenticators in persistent storage based on a stable user ID (read: a primary key for user table). User names (emails, nicknames, etc.) often change during user's lifecycle, but IDs don't.

Currently it's not possible to do this without doing an extra lookup to find user ID based on username (as authenticatorUpdater).

#### Contribution

Already have a PR in the works at https://github.com/mnylen/vertx-auth/pull/1/files