Add OpCode method to autogenerated operation types

[jn9e9]

In order to reduce chance of picking incorrect opcode in the operations_client.go, define it in an OpCode method attached to each request type.
Would it be possible to deduce the opcode from the type of request? As in have a method on each type of request to return the corresponding opcode? That would remove one parameter to operation

Originally posted by @hug-dev in #28 (comment)