Transfer: Registry: Enable plain HTTP

Currenlty transfer service doesn't handle plain HTTP connection.
This commit fixes this issue by propagating
`(core/remotes/docker/config).HostOptions.DefaultScheme` from client to the
transfer service.
This commit also fixes ctr to use this feature for "--plain-http" flag.

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

api/next.pb.txt

@@ -7468,6 +7468,13 @@ file {
       type: TYPE_STRING
       json_name: "hostDir"
     }
+    field {
+      name: "default_scheme"
+      number: 4
+      label: LABEL_OPTIONAL
+      type: TYPE_STRING
+      json_name: "defaultScheme"
+    }
     nested_type {
       name: "HeadersEntry"
       field {