Make WebAuthnAuthentication Serializable

[jzheaux]

WebAuthnAuthentication implements Serializable, but cannot be serialized due to PublicKeyCredentialUserEntity, one of its members, not being serializable.

[justincranford]

I reported a similar issue, but more broadly it affects registration too.
#16328

WebAuthn + Redis doesn't work out-of-the-box...

1. WebAuthn classes lack Serializable interface, and Redis defaults to JdkSerializationRedisSerializer.
2. WebAuthn has mixins for GenericJackson2JsonRedisSerializer, but I had to add 12 extras because bundled mixins are partially incomplete or missing.

Workarounds to create an ObjectMapper for GenericJackson2JsonRedisSerializer which can serialize the WebAuthn registration and authentication request objects.
#16328 (comment)

Workarounds to configure Redis with GenericJackson2JsonRedisSerializer.
#16328 (comment)