change AuthenticatorTransport to be other than an enum

[equalsJeffH]

we want to add values to AuthenticatorTransport without non-congnizant UAs throwing errors on values they are unaware of. change it to a sequence of strings? see also PR #1264

[jcjones]

Just to be clear, right now the Web Authentication spec only allows values for AuthenticatorTransport that are in the level 1 spec. Permitting other values is a violation of WebIDL's definition of enumeration

A fix for this would be to change PublicKeyCredentialDescriptor's transports sequence to one of DOMString and update the algorithm to note that each string should be coerced into a transport type known by the implementation, or ignored if unknown.

dictionary PublicKeyCredentialDescriptor {
    required PublicKeyCredentialType      type;
    required BufferSource                 id;
    sequence<DOMString>                   transports;
};

[jcjones]

Also note, this is a breaking change, in that if updates aren't done in the right sequence, we'll have webcompat issues.

[jcjones]

It appears that Chromium has already made the breaking change, so we do have an active webcompat issue. Cite: https://security.googleblog.com/2019/08/making-authentication-even-easier-with_12.html