idea: kv adapter's scan should return an iterator or stream instead

[Xuanwo]

We return Vec<String> to make it simple, but in some usecases, users doen't want to have the whole content.

[suyanhanx]

Like the reader, with poll based?

[Xuanwo]

Like the reader, with poll based?

Maybe use async_trait is enough since we won't expose them as public API. After we bump to rust 1.74, we can use async in trait directly without async_trait.