tor-proto: Add (unused) `HopLocation` and `TargetHop`

This is a small MR to add prototypes of the HopLocation and TargetHop types. I decided to do this as its own MR so that the types will exist somewhere in the code and can be referenced.

I'm currently working on updating tor-proto to use these types. But this will be without the JoinPoint variant and Leg variant as there's no way to handle these variants yet, and I figure it's better to not include them than to have to match on them and panic.

Part of #1857.