use attribute instead of generic for routed_str_topic

[tigroo]

Into the example, collector routed str topic declare statically 5 potential levels:

its-client/rust/examples/collector/collector_str_topic.rs

Line 62 in a851f99

pub enum CollectorStrTopic {

It could be replaced by an attribute, to allow the Packet declaration to compile AND to avoid to redeclare a local type:

its-client/rust/examples/collector/main.rs

Line 233 in a851f99

let packet = Packet::<CollectorStrTopic, String> {