feat: update Duration and Timestamp to match proto specs (#517)

Updates the `Duration` and `Timestamp` types in unionlabs to be in line
with their proto specs.

TODO:

- [x] More tests for all conversions
- Added basic fuzzing (start of #409). We can look into using
[kani](https://github.com/model-checking/kani) as well.
- [x] Improve docs (these two types likely could use a module doc
explaining why they are the way they are)
- Copied some of the docs from the proto types; this can be expanded
upon in the future.

Also fixed a few clippy warnings because they were annoying me