Remove unnecessary hashing from IDs (#2998)
## Summary
In all of these ID types, we pass values to `cache_key::digest` prior to
passing to `DistributionId` or `ResourceId`. The `DistributionId` and
`ResourceId` are then hashed later, since they're used as keys in hash
maps.
It seems wasteful to hash the value, then hash the hashed value? So this
PR modifies those structs to be enums that can take one of a fixed set
of types.
