Transfer: Registry: Enable to use registry configuration diretory

Currently transfer service isn't aware of configurations of hosts directory and
ctr's `--hosts-dir` doesn't work.
This commit fixes this issue by using `config.ConfigureHosts` instead of
`docker.ConfigureDefaultRegistries`.
This commit also fixes ctr to use this feature for "--hosts-dir" flag.

Signed-off-by: Kohei Tokunaga <ktokunaga.mail@gmail.com>

api/next.pb.txt

@@ -7461,6 +7461,13 @@ file {
       type_name: ".containerd.types.transfer.RegistryResolver.HeadersEntry"
       json_name: "headers"
     }
+    field {
+      name: "host_dir"
+      number: 3
+      label: LABEL_OPTIONAL
+      type: TYPE_STRING
+      json_name: "hostDir"
+    }
     nested_type {
       name: "HeadersEntry"
       field {