Refacto/improve code quality

[tchapacan]

The code has been pushed in early stage and i'm a beginner in rust, so I'm sure there is a plenty of room for improvement.

I didn't want to overengineer the codebase for such a simple usecase, but some part can be improve and discuss:

• There is a lot of long function in main.rs like here https://github.com/tchapacan/livebox-exporter-rs/blob/main/src/main.rs#L176-L199 (and elsewhere as well)
• This part work as intended but the implementation can be a little naive: https://github.com/tchapacan/livebox-exporter-rs/blob/main/src/main.rs#L124-L162

Seek if we can use leverage a proper datastructure / type and iterate through it to pass parameter while using the querying method?

• In client.rs separate the logic of building the different type of query (post, post_authenticated, login, logout) and the actual query to retrieve metrics (get_wan_config, get_status, get_devices, etc..), some how like a service <==> adapter "pattern"

• If you are confident enough in rust, dig deeper in the structure of the project (module, trait, "class") etc