Fix veloren-server compilation

As veloren-server enables plugin feature automatically, it results in
veloren-common-state inherit this feature, which enables
common/state/plugin/mod.rs which asks for common::assets function that
is enabled only if plugin feature is enabled, but because
veloren-common-state doesn't depend on common::assets, this feature is
kind of lost half-way.

This commit fixes this by adding explicit optional dependency on
common-assets in common-state that is enabled by plugin feature.