[object_store] Runtime Agnostic Retry Machinery

[tustvold]

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

Whilst the HttpClient abstraction allows spawning work to a separate tokio runtime, see apache/arrow-rs#7253, the retry machinery itself also makes use of tokio - see here.

This forces the calling context to be a tokio content.

Describe the solution you'd like

I think we should make it possible to use object_store in a non-tokio runtime.

Describe alternatives you've considered

We could simply disable backoff and rety if not called from a tokio runtime.

Additional context