spec: added alpha_xxx options

Enumerations, fields, messages, methods, and services support
`alpha_xxx` designations, indicating that they are part of an
experimental feature that may never evolve to "stable" status.

csi.proto

@@ -8,10 +8,45 @@ import "google/protobuf/wrappers.proto";
 
 option go_package = "csi";
 
+extend google.protobuf.EnumOptions {
+  // Indicates that this enum is OPTIONAL and part of an experimental
+  // API that may be deprecated and eventually removed between minor
+  // releases.
+  bool alpha_enum = 1060;
+}
+extend google.protobuf.EnumValueOptions {
+  // Indicates that this enum value is OPTIONAL and part of an
+  // experimental API that may be deprecated and eventually removed
+  // between minor releases.
+  bool alpha_enum_value = 1060;
+}
 extend google.protobuf.FieldOptions {
   // Indicates that a field MAY contain information that is sensitive
   // and MUST be treated as such (e.g. not logged).
   bool csi_secret = 1059;
+
+  // Indicates that this field is OPTIONAL and part of an experimental
+  // API that may be deprecated and eventually removed between minor
+  // releases.
+  bool alpha_field = 1060;
+}
+extend google.protobuf.MessageOptions {
+  // Indicates that this message is OPTIONAL and part of an experimental
+  // API that may be deprecated and eventually removed between minor
+  // releases.
+  bool alpha_message = 1060;
+}
+extend google.protobuf.MethodOptions {
+  // Indicates that this method is OPTIONAL and part of an experimental
+  // API that may be deprecated and eventually removed between minor
+  // releases.
+  bool alpha_method = 1060;
+}
+extend google.protobuf.ServiceOptions {
+  // Indicates that this service is OPTIONAL and part of an experimental
+  // API that may be deprecated and eventually removed between minor
+  // releases.
+  bool alpha_service = 1060;
 }
 service Identity {
   rpc GetPluginInfo(GetPluginInfoRequest)