Make `GetOptionsExt` publicly usable

[gruuya]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
Ideally, to implement the new ObjectStore::get_opts for other object stores based on reqwest clients, we could simply call GetOptionsExt::with_get_options on the RequestBuilder, as is done in the object_store crate itself.

This is not currently possible (I assume inadvertently as the trait itself is public?), since the containing client mod is currently private;

Describe the solution you'd like
Make the trait public/exportable beyond the object_store crate.

Describe alternatives you've considered
Repeat the logic from GetOptionsExt::with_get_options.

Additional context
Over at Seafowl we have a custom HTTP object store implementation for legacy reasons.

[tustvold]

These extension traits are intentionally private, as they're really just implementation details, that being said GetOptionsExt seems fairly harmless to make public

[Blizzara]

Hey @tustvold, somewhat related question - would you consider making the whole client mod public? We have our own custom ObjectStore implementation that's backed by an S3 api with some extras, but I think we could reuse most/all of the client. Alternatively I can also just make a copy of it for us, but wanted to check here first :)

[tustvold]

I don't think we could make it public in its current form, the API surface is simply too large and not really designed around API stability.

I think probably we would need something like apache/arrow-rs#6056 to provide a coherent contract, which we could then provide a default implementation of.

[Blizzara]

I actually ended up copying (and slightly adapting) just the Backoff and Retry implementations. The client itself wouldn't provide us value, but those seemed general enough.