Decouple ObjectStore from Reqwest / Generic HTTP Client Support

[tustvold]

This is a high-level tracking ticket and is not really actionable

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

We currently depend extensively on reqwest for our HTTP client. This has served us very well, however, there has been some community interest in being able to either use different HTTP clients (#6051), or alter the behaviour of the client in non-trivial ways (#5990).

Describe the solution you'd like

Users provide a custom client factory that takes a ClientOptions and returns some generic HTTP Client abstraction. There are a lot of unknowns about how this would work, in particular for request retries, streaming requests, or more advanced configurations around TLS, proxies, etc... but this would all need to be worked out

Describe alternatives you've considered

This could be considered out of scope and too much scope creep for this crate, users with more advanced requirements should instead provide their own implementation of the ObjectStore trait.

Additional context

[alamb]

Another alternative might be to help improve reqwest upstream so that it doesn't require tokio

[tustvold]

The maintainers have at least previously indicated they do not wish to support runtimes other than tokio, I probably should have included this in the additional context - seanmonstar/reqwest#719