Use view_functions for duster runtime api

[dmoka]

View functions are a more user friendly and easier way to declare runtime apis.

Once we upgraded to stabe2503, we can start using it: https://github.com/paritytech/polkadot-sdk/releases/tag/polkadot-stable2503

(Here is the required functionality merged to stable2503: paritytech/polkadot-sdk#4722)