node: new concrete type for seed node implementation (#7521)

Defines a different concrete type that satisfies the service interface for a seed node.
update the seed node unit test to assert the new type.

Fixes #6775

node/node_test.go

@@ -556,7 +556,7 @@ func TestNodeNewSeedNode(t *testing.T) {
 	t.Cleanup(ns.Wait)
 
 	require.NoError(t, err)
-	n, ok := ns.(*nodeImpl)
+	n, ok := ns.(*seedNodeImpl)
 	require.True(t, ok)
 
 	err = n.Start(ctx)