Unify runtime abstractions

[AdamGS]

There are multiple places where we want to specialize for different async runtimes, or where we make assumptions about the availability of a specific runtime (like in Datafusion, where we assume we're running in a tokio one). The IoDispatcher covers some of these cases, but its a bit too low level which makes us hack around it.
I think the general things we want to cover are:

1. Spawn any work (both futures and streams)
2. Disk and network IO (probably includes centralizing our current IO traits)
3. Consistent behavior for features

Other crates do similar things, Examples include:

• hyper
• opendal
• parquet-rs

[Xuanwo]

By the way, are you interested in adding OpenDAL support to vortex-io? I'm willing to contribute.

[AdamGS]

Yeah that would be great! I think our IO traits are still changing and having more examples will be good + opendal seems pretty stable so I don't expect it to be hard to maintain.