[go binding] peer.TransactionAction#Header

```
// TransactionAction binds a proposal to its action. The type field in the header dictates the type of action to be applied to the ledger.
type TransactionAction struct {
// The header of the proposal action, which is the proposal header
Header []byte `protobuf:"bytes,1,opt,name=header,proto3" json:"header,omitempty"`
// The payload of the action as defined by the type in the header For
// chaincode, it's the bytes of ChaincodeActionPayload
Payload []byte `protobuf:"bytes,2,opt,name=payload,proto3" json:"payload,omitempty"`
...
}
```
As documented, the header should be of the `proposal header` type, which I cannot find which is the type it refers to.