Allow for `config.authenticator` to be optional

[magom001]

Currently AttestationChallengeBuilder.setAuthenticator throws an error if config.authenticator is not cross-platform or platform

This field is not mandatory. If not provided a browser prompt is presented (@see https://webauthn.io/).

I suggest to remove

webauthn/src/AttestationChallengeBuilder.js

Lines 157 to 161 in d8d63d2

	const values = Object.values(Dictionaries.AuthenticatorAttachment)
	if (!values.includes(authenticatorAttachment)) {
	throw new Error(`Invalid AuthenticatorAttachment value. Must be one of "${values.join('", "')}".`)
	}