[CAIP-196] Timestamp precision

[chunningham]

CAIP-196 currently defines timestamps as integers. 99% of the time this is what they will be, however the JWT spec defines timestamps as "NumericDates" which can have fractional values. The UCAN spec similarly bases itself on the JWT spec, this issue mentions that they 'should be the same as in JWT, because these are JWT fields, although the spec itself mentions integers. Hypothetically, what should happen if a UCAN with exp: 2.5` is translated into a CACAO?