JSON-RPC: Review {instrument,country,...}Id usage

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Some APIs work with serialized data (skill level, directory server status), some other APIs return or expect internal ids such as instrumentId or countryId.

**Has this feature been discussed and generally agreed?**

No.

**Describe the solution you'd like**

We should define how this is supposed to be.
1. Do we try to be user-friendly and serialize everything to a proper string so that the API and the implementation are rather disentangled?
2. Do we keep implementation overhead low and expose internal ids via the API?
3. Do we decide on a case-by-case basis?

As an API-person, I tend towards (1). It's also becoming obvious that making implementation details such as the countryId being a Qt5 QLocale::Country has downsides.

**Describe alternatives that have been considered**

See above.

Inizia esaminando le API JSON-RPC esistenti che serializzano skill level e directory server status, insieme alle API che usano instrumentId o countryId. Confronta la gestione attuale di questi valori e stabilisci se l’API debba esporre stringhe, ID interni o rappresentazioni caso per caso; il lavoro è completato quando è stata concordata una specifica coerente.

Scritto dal modello di indicizzazione a partire dal testo della issue.