Support Sled key-value store as a Store

[czocher]

I recently came across Sled when I searched for other possibilities for persistent application state. I think it would be nice to support it as an alternative for sqlx.

[maxcountryman]

Agree this looks cool! Just skimming the project, it looks like they have some kind of async story so that bodes well.

I'm open to adding this.

[czocher]

I asked the authors about any special handling we need to perform.

[czocher]

Hello,
I had a discussion on the sled Discord and it seems that this project should be considered dead and replaced by marble. I'll close this issue since there's no reason to implement support for an abandoned project. We can check if we can implement marble support in another task.