Register local content plugin from separate package

Update the local content plugin to register itself in a consistent way
as other plugins. This also allows the separate package to define its
own configuration more cleanly.

Signed-off-by: Derek McGowan <derek@mcg.dev>

integration/build_local_containerd_helper_test.go

@@ -36,6 +36,7 @@ import (
 
 	_ "github.com/containerd/containerd/api/types/runc/options"
 	_ "github.com/containerd/containerd/v2/core/runtime/v2"
+	_ "github.com/containerd/containerd/v2/plugins/content/local/plugin"
 	_ "github.com/containerd/containerd/v2/plugins/cri/images"
 	_ "github.com/containerd/containerd/v2/plugins/cri/runtime"
 	_ "github.com/containerd/containerd/v2/plugins/diff/walking/plugin"