spec: experimental service: ObjectStore.

Mock up of an experimental object storage API. Note the use of the
`alpha_service` marker option for the new gRPC service, as well as the
additional `alpha_xxx` markers for the new RPCs and top-level
request/response messages.

csi.proto

@@ -127,6 +127,20 @@ service Node {
   rpc NodeGetInfo (NodeGetInfoRequest)
     returns (NodeGetInfoResponse) {}
 }
+
+service ObjectStore {
+  option (alpha_service) = true;
+
+  rpc ObjectStoreRead(ObjectStoreReadRequest)
+    returns (ObjectStoreReadResponse) {
+      option (alpha_method) = true;
+  }
+
+  rpc ObjectStoreWrite(ObjectStoreWriteRequest)
+    returns (ObjectStoreWriteResponse) {
+      option (alpha_method) = true;
+  }
+}
 message GetPluginInfoRequest {
   // Intentionally empty.
 }
@@ -236,6 +250,9 @@ message PluginCapability {
       // Indicates that the plugin supports data lineage reporting for
       // volumes.
       VOLUME_LINEAGE = 1;
+
+      // Indicates that the plugin supports the object storage service.
+      OBJECT_STORE = 2;
     }
 
     Type type = 1; // REQUIRED
@@ -1452,3 +1469,50 @@ message NodeExpandVolumeResponse {
   // The capacity of the volume in bytes. This field is OPTIONAL.
   int64 capacity_bytes = 1;
 }
+message ObjectStoreReadRequest {
+  option (alpha_message) = true;
+
+  message DataSelector {
+    message Range {
+      int64 offset = 1;
+      int64 length = 2;
+    }
+
+    repeated Range range = 1;
+  }
+
+  string location = 1;
+  map<string, string> secrets = 2 [(csi_secret) = true];
+
+  oneof kind {
+    bool metadata_only = 3;
+    DataSelector selector = 4;
+  }
+}
+
+message ObjectStoreReadResponse {
+  option (alpha_message) = true;
+
+  message Segment {
+    int64 offset = 1;
+    bytes value = 2;
+  }
+
+  repeated Segment segments = 1;
+}
+message ObjectStoreWriteRequest {
+  option (alpha_message) = true;
+
+  string location = 1;
+  map<string, string> secrets = 2 [(csi_secret) = true];
+
+  oneof action {
+    bytes data_append = 3;
+    bytes data_overwrite = 4;
+    bool remove = 5;
+  }
+}
+
+message ObjectStoreWriteResponse {
+  option (alpha_message) = true;
+}