Prevent using field name "creator" and "sender" when scaffolding a type/packet/message

[lumtis]

• When scaffolding a type or a message, a field Creator is automatically added that represents the signer of the message scaffolded.

If we scaffolding a type with a manual creator field:

blog/tx.proto:24:10: "creator" is already defined in "tuto.planet.blog.MsgCreateSentPost".
blog/tx.proto:37:10: "creator" is already defined in "tuto.planet.blog.MsgUpdateSentPost".

We must return an explicit error and prevent creator field for type and message

• We must prevent sender field for packet

• [EDIT] We must also prevent using id and index respectively for CRUD and for indexed types