AlterType, fix payload serialization

[sl-at-ibm]

This PR changes the resulting serialization for AlterTypePayload as follows: the operation layer is removed (the Data API does not expect it).

So instead of having e.g.

{
    "alterType": {
        "name": "member",
        "operation": {
            "add": {
                "fields": {
                    "email": {
                        "type": "text"
                    },
                    "credits": {
                        "type": "int"
                    }
                }
            }
        }
    }
}

we now end up with the working json:

{
    "alterType": {
        "name": "member",
        "add": {
            "fields": {
                "email": {
                    "type": "text"
                },
                "credits": {
                    "type": "int"
                }
            }
        }
    }
}

To achieve this, some of the logic around marshaling of the AlterTypePayload are applied (including the introduction of a "key name" string method to the alter type operation interface for building the appropriate payload).

(Besides the altered unit test, this branch is tested manually by altering a type in both add and rename ways, successfully).

Reference Slack conversation: here.