Implement version specific types for status 'omitted' methods

[tcharding]

Currently we have a status 'omitted' that was meant to imply that we intentionally omitted adding a type. For example if method returns a std lib type

$ bt17 help pruneblockchain
pruneblockchain

Arguments:
1. "height"       (numeric, required) The block height to prune up to. May be set to a discrete height, or a unix timestamp
                  to prune blocks whose block time is at least 2 hours older than the provided timestamp.

Result:
n    (numeric) Height of the last block pruned.

This has the problem that later versions of the method might change and we will not catch it.

It would be better to add a version specific type, implement in the client, and an integration test that calls the method. Even for null, which is currently does as follows:

                match self.call("submitblock", &[into_json(hex)?]) {
                    Ok(serde_json::Value::Null) => Ok(()),
                    Ok(res) => Err(Error::Returned(res.to_string())),
                    Err(err) => Err(err.into()),
                }

Note that doing this also requires updating the method in verify (e.g., verify::metho::v17) to use Method::new_no_model.