plugin_macros: add a new macro to generate the init plugin and import the necessary module

[vincenzopalazzo]

With the following PR https://github.com/laanwj/rust-clightning-rpc/pull/36 we introduce the macros to register a new notification, and this cause a problem because multiple macros can import the same stuff, so a possible solution is to implement a plugin! macros where all these standard import are included. We can remove it from the single macros.

Required a little bit of rust macros knowledge

[vincenzopalazzo]

Some more steps are done, but it requires some more work, like the way to easy parsing and configures the plugin options.

This is a good time to implement a derive macro