[webauthn] Add notion of internal/external to options

leshi has just created a new issue for 
https://github.com/w3c/webauthn:

== Add notion of internal/external to options ==
Add notion of internal/external to options so that Caller can indicate
 that it only wants to talk to internal authenticators.

Example:
```
dictionary CredentialOptions {
    unsigned long                       timeoutSeconds;
    sequence < CredentialDescription >  excludeList;
    WebAuthnExtensions                  extensions;
    Attachment                          attachment;
};

enum Attachment {
    "INTERNAL",
    "EXTERNAL"
}

Please view or discuss this issue at 
https://github.com/w3c/webauthn/issues/213 using your GitHub account