Make TimeUnit and IntervalUnit Copy

[hiltontj]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

The TimeUnit and IntervalUnit types in the arrow-schema crate are not Copy, but they are enums with only unit variants, so they should be cheaply Copy.

Describe the solution you'd like

Derive Copy for these types.

Describe alternatives you've considered

Can cheaply use clone, which is not the end of the world, but unless there is a reason to keep these types not-Copy, this would be an ergonomic improvement.

Additional context

N/A.

[alamb]

label_issue.py automatically added labels {'arrow'} from #5840