feat(swagger): agent, add gs endpoint abstractions

As we need to enable testing against mock gcs server,
we also need to get configurations of rclone and scylla
gcs endpoints, so that we can match them.

The rclone endpoint property was added in the last commit.
The only new scylla property added with introduction of gcs
endpoints is the type property taken from:
https://github.com/scylladb/scylladb/blob/master/docs/dev/object_storage.md#configuring-gcp-storage-access.

Refs #4597

Author: Michal-Leszczynski

v3/swagger/agent.json

@@ -1748,7 +1748,11 @@
             "gcs": {
               "type": "object",
               "x-nullable": false,
-              "properties": {}
+              "properties": {
+                "endpoint": {
+                  "type": "string"
+                }
+              }
             },
             "azure": {
               "type": "object",
@@ -1798,6 +1802,9 @@
         },
         "iam_role_arn": {
           "type": "string"
+        },
+        "type": {
+          "type": "string"
         }
       }
     }