Sled scan KV query support

[joepio]

Hi there! First off, thank you all for creating this absolutely amazing project. It's a solution to a problem I did not expect to find. I'm working on an open source graph database / notion alternative called atomic-server, and want users to choose between Sled and other KV stores. Also I'd like to use S3 instead of a local filesystem, so OpenDAL is useful in multiple ways for my project.

I use Sled's rangequeries, but OpenDAL only has support for scan. I think the difference is trivial - if I'm understanding this correctly, the only difference is that range has an explicit upper bound.

I suppose that's not a problem, so if scan is implemented for sled, I'll be very happy.

There seems to be a scan implementation in OpenDAL for sled, but it's not checked in the docs.

[Xuanwo]

Yes, our feature docs for sled is out-dated. Let me fix it.