Make ObjectStoreScheme in the object_store crate public

[orf]

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

I want to parse an object store URL into a type. Using parse_url and parse_url_opts casts the result into a Box<dyn ...> which makes it impossible to get the underlying type from.

Describe the solution you'd like

Making ObjectStoreScheme public will allow me to do:

let (scheme, path) = ObjectStoreScheme::parse("s3://foo/bar")

[drmorr0]

+1, I want to support all the different cloud providers in my app, but I need more fine-grained control over how the ObjectStore is constructed, so I can't just use parse_url -- having this public would be very useful, I literally just came here to submit a PR to do the same thing :)

[alamb]

label_issue.py automatically added labels {'object-store'} from #5912