unknown import: `ethereum::ethereum.call` has not been defined

Summary

This happens when a file data source handler uses contract calls directly or is in the same AssemblyScript file as other handlers which use contract calls. If the contract call is done by a different handler in the same file, this can be fixed by putting the file data source handler in its own file, which is best practice.

Potential Fix

Contract calls in file data source handlers could be enabled by somehow passing the ethereum.call host fn when instantiating an offchain data source, relevant code:
https://github.com/graphprotocol/graph-node/blob/92670f27d93ed08a6e83805ad644d9004d4d77ee/runtime/wasm/src/host.rs#L137-L141

Parti da runtime/wasm/src/host.rs, intorno alle righe 137-141, e traccia il modo in cui vengono istanziate le origini dati offchain. Verifica in che modo la funzione host ethereum.call mancante influisce sugli handler delle origini dati dei file, quindi determina se può essere passata durante l'istanziazione. Il lavoro è completato quando le chiamate ai contratti non producono più l'errore unknown import negli scenari degli handler descritti.