sources/ostree: set contenturl when pulling from remote

If a contenturl is specified, the url is used only for metadata. This is
useful when the actual content is hosted separately.

sources/org.osbuild.ostree

@@ -38,6 +38,10 @@ SCHEMA = """
                 "type": "string",
                 "description": "URL of the repository."
               },
+              "contenturl": {
+                "type": "string",
+                "description": "content URL of the repository."
+              },
               "gpgkeys": {
                 "type": "array",
                 "items": {
@@ -107,6 +111,9 @@ class OSTreeSource(sources.SourceService):
         if not gpg:
             remote_add_args = ["--no-gpg-verify"]
 
+        if "contenturl" in remote:
+            remote_add_args.append(f"--contenturl={remote['contenturl']}")
+
         if remote.get("secrets", {}).get("name") == "org.osbuild.rhsm.consumer":
             secrets = Subscriptions.get_consumer_secrets()
             remote_add_args.append(f"--set=\"tls-client-key-path={secrets['consumer_key']}\"")