enhance: split config from server package

The github.com/containerd/containerd/services/server has a lot of
dependencies, like content, snapshots services implementation and
docker-metrics.

For the client side, it uses the config struct from server package
to start up the containerd in background. It will import a lot of
useless packages which might be conflict with existing vendor's package.

It makes integration easier with single config package.

Signed-off-by: Wei Fu <fuweid89@gmail.com>

services/server/server_unsupported.go

@@ -18,8 +18,12 @@
 
 package server
 
-import "context"
+import (
+	"context"
 
-func apply(_ context.Context, _ *Config) error {
+	srvconfig "github.com/containerd/containerd/services/server/config"
+)
+
+func apply(_ context.Context, _ *srvconfig.Config) error {
 	return nil
 }