[Feature Request / Contribution Proposal] working with internet drafts

Hi, and first of all thanks for the crate, it great, love it.

When working with internet drafts there are very often a lot of pending/requested IANA codepoints that are not within the private range and hence cannot be used within the `ClaimSetBuilder` which is a very convenient way to build a Cose object.

One have to resort to do the CBOR serialization by hand on the side to bypass [this line](https://github.com/google/coset/blob/442392251948419244e50dac052500702556c906/src/cwt/mod.rs#L195).

My proposal is to add another variants like such:

```rust
pub enum RegisteredLabelWithPrivate {
PrivateUse(i64),
InternetDraft(T),
Assigned(T),
Text(String),
}

#[derive(Clone, Debug, Eq, PartialEq)]
pub enum RegisteredLabel {
InternetDraft(T),
Assigned(T),
Text(String),
}
```

And an associated `ClaimSetBuilder::internet_draft_claim` that would let pass arbitrary labels.

What do you think ?