Ability to configure out client/server support via feature flags

[kanpov]

I.e. put the sftp client behind client feature and server behind server feature, so that client-only applications don't need to pull in server code (or have it be optimized away by LTO at the cost of an increase in linking time) and vice versa for server-only applications. If both are enabled as default features, this wouldn't be a breaking change.

Would this be acceptable?

[AspectUnk]

Sure. PR welcome