[mqtt client] Support subtree `get()`

[jordens]

Support getting (request to republish) a subtree. Host publishes the empty message to a topic that would otherwise result in miniconf::Error::PathTooShort. In the mqtt client the pseudocode would be something like: try pub(get(path)); if PathTooShort: for p in iter.filter(startswith path): ... pub(get(p))