Add interface for accessing transports

[santiagorodriguez96]

Summary

Step 23 of the Registering a New Credential section of the WebAuthn Level 2 spec states:

It is RECOMMENDED to also:

Associate the credentialId with the transport hints returned by calling credential.response.getTransports(). This value SHOULD NOT be modified before or after storing it. It is RECOMMENDED to use this value to populate the transports of the allowCredentials option in future get() calls to help the client know how to find a suitable authenticator.

This PR adds the necessary interface for relying parties to easily access to this value, by doing:

WebAuthn::Credential.from_create(credential_params).response.transports # => ["internal"]