Use enums instead of Strings where possible values are known

[laanwj]

The API could make more use of enums where the possible values are known; for example the addresstype parameter to newaddr, but also in returned structures. This has to be weighted against flexibility, though, in case the API is extended later.

[vincenzopalazzo]

ack about this idea, I really love to have also a generic method to make some building blocks for the user to make a custom call for all the c-lightning methods.

This can be particularly useful when a user has some retro compatibility problem.

An example can be fn call_rpc(name_command: String, params: HashMap<String, Any>)