feat: Implement Serialize/Deserialize for UCAN

[matheus23]

Also: Take impl AsRef<str> in some places instead of String (and standardize on specifying it as an impl-in-place).
Also also: Encode EmptyCaveat as {} instead of null

[QuinnWilton]

Ah, thank you! The issue with EmptyCaveat must have been a subtle regression when I changed things from struct EmptyCaveat {} to struct EmptyCaveat;

[matheus23]

Yeah, I think so too.
However, this additionally also fails parsing if there's any fields in the object at all. I think EmptyCaveat {} previously accepted objects with fields, it would simply ignore them IIRC.