Consider switching `Publisher` to be a `RootModel`

[woodruffw]

I initially proposed this in #43, but it's somewhat disruptive/requires access through a .root attribute, which is annoying.

[facutuesca]

Looking a bit into this, the changes to our code in order to use RootModel seem minimal enough. I guess the problem would be for when a user of the library accesses AttestationBundle.publisher and it doesn't behave like a Publisher unless he adds .root. Is that right?

[woodruffw]

I guess the problem would be for when a user of the library accesses AttestationBundle.publisher and it doesn't behave like a Publisher unless he adds .root. Is that right?

Yep, exactly. This is probably not worth pursuing given that, especially since it'll be unfamiliar to most users who have experience with Pydantic.