fix: allow passing gax instance to client constructor

protos/google/spanner/v1/type.proto

@@ -163,4 +163,12 @@ enum TypeAnnotationCode {
   // [NUMERIC][google.spanner.v1.TypeCode.NUMERIC] when a client interacts with PostgreSQL-enabled
   // Spanner databases.
   PG_NUMERIC = 2;
+
+  // PostgreSQL compatible JSONB type. This annotation needs to be applied to
+  // [Type][google.spanner.v1.Type] instances having [JSON][google.spanner.v1.TypeCode.JSON]
+  // type code to specify that values of this type should be treated as
+  // PostgreSQL JSONB values. Currently this annotation is always needed for
+  // [JSON][google.spanner.v1.TypeCode.JSON] when a client interacts with PostgreSQL-enabled
+  // Spanner databases.
+  PG_JSONB = 3;
 }