…est pushed

Current implementation hides many data available from authenticators
behind the private methods. It is useful to expose them to users of this
gem, i.e. logging, auditing, and debugging.

I believe this gem should provide a low-level API for RP
implementations.

To allow RP to check signature count mismatch

https://www.w3.org/TR/2018/CR-webauthn-20180807/#sign-counter

OpenStruct doesn't need any attributes to be declared beforehand. But
this reduces readability, even all attributes used are known.

We have to stop using keyword arguments to initialize Credential struct,
but we can back later once we dropped Ruby 2.4 support by Struct `:keyword_init`.

Closes #72

RP ID may be different from request origin; i.e. client may override,
or the appid extension may give legacy app id as a RP ID.

In most cases, rp_id can be expected to be a host part of request origin
URI. So this patch allows `valid?` methods to take optional `rp_id`, then the
methods handle the explicitly given one as a RP ID, instead of a host of
origin URI, when it's given.

refactor: make AttestedCredentialData::Credential a Struct

feat: Make methods public to expose data available

Parse signCount and expose at AuthenticatorData#sign_count

docs: Fix functional typo in `README.md`: publick_key -> public_key.

feat: Accept rp_id explicitly on *Response#valid?