chore: remove duplicated code between union and cosmos (#1587)

- there was a lot of copy-pasted code between union and cosmos in
voyager, this has been deduplicated now
- added helpers a la `Fetch::specific` to more tersely construct
`{LightClient,Chain}Specific{Data,Fetch,Aggregate}`, removing lots of
nested expressions and `.into()` calls
- union now uses `TrustedValidators` and `UntrustedValidators` instead
of just `Validators`, mirroring cosmos, so that the aggregation is not
dependant on the order of the data in the aggregation queue
- use `#[apply(msg_struct)]` in a few more places
- use more fully qualified paths in macros to reduce imports in files
they're called in
- use `id()` instead of `Identified::new()`
- should probably remove the `Identified::new` constructor at this
point, will do in a future PR

i meant to break this up into two commits, but i accidentally amended
instead of committed. oops