Re: [webauthn] PublicKeyCredentialDescriptor.id and PublicKeyCredentialEntity.id type differ

And it makes things far more elegant:

Before
```js
...
user: {
    id: "1098237235409872"
    name: "john.p.smith@example.com",
    displayName: "John P. Smith",
    icon: "https://pics.acme.com/00/p/aBjjjpqPb.png"
  },
...

...
var acceptableCredential1 = {
    type: "public-key",
    id: encoder.encode("1098237235409872")
};
...
```

After
```js
...
user: {
    id: "1098237235409872"
    name: "john.p.smith@example.com",
    displayName: "John P. Smith",
    icon: "https://pics.acme.com/00/p/aBjjjpqPb.png"
  },
...

...
var acceptableCredential1 = {
    type: "public-key",
    id: "1098237235409872"
};
...
```

-- 
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